The M Twins' power is great. They possess a high degree of telepathy, able to use it in limited offensive capabilities such as and can open inter-dimensional gateways into alternate planes of existence. As revealed in a flashback in Generation X #40, the twins opened a doorway to another dimension to imprison their villainous brother Emplate as a punishment for his suspected (later discovered to be actual) involvement in their mother's death and their sister M's disappearance.
Nicole and Claudette also possess a rare power called "gestalt." With it, they can (and have) physically merged with with Marius (Emplate), Monet (M), Hollow or any combination thereof. Each combination has a new, different personality and set of powers. A simple example of a gestalt: When they were masquerading as their older sister Monet, they possessed many abilities, not all of which may have actually existed in the real Monet.
The M Twins, Claudette and Nicole, first appeared, posing as their older sister Monet, in Uncanny X-Men issue 316 (1994). It was only revealed later that this Monet was actually the twin girls that had morped into Monet. The twin's first appearance as themselves was in Generation X issue 31 (1997).
Major Story Arcs
During the Phalanx Arc, The M-twins are in a limousine. With a colonel who is complaining about Monet not talking. She tells her if she say anything her dad might send her to get help from Xavier in the states who deal with mutants. The car then turns off and nothing in the car works. It crashes against a rail. In front of the car is a phalanx who absorbs the driver. It goes to the back of the car to get Monet, the Colonel throws Monet out of the car and tells her to run for her life. She then shoots the phalanx to no result only to be absorbed by it. Monet just stays in the same place. The phalanx tries to make a biocentric scan of Monet but he says there is too much intense psionic energy blocking him from doing it. The phalanx then states that the being might be the most powerful of the next generation of homo superior. Monet is then seen in a chamber with Skin, Paige, Cannonball and Blink. They still make no contact with the other people. She then tells them to be quiet. Monet is was making a device that would get them out this device showed that Cannonball was part of the phalanx. The phalanx then turns to stone. Monet already knew Cannonball was a phalanx. And tells the others there are few things she doesn’t know. She then tells them that the phalanx placed him in here as a spy. She then tells them the device she made also put a illusion to the other phalanx that they are still in the room while they escape.
(When saying M or Monet, it is the M-Twins as Monet)
During Generation X the Twins were masquerading as Monet and nobody knew. M-Twins first appeared as the kids were being gathered together to go to school. She makes a comment on how mutants shouldn't hide their abilities and should be proud of them. When she comes in she asks Sean Cassidy to get her bags, he says yes to three trucks full of bags. They are the first ones who want to go to the training session. M-Twins like to climb trees even though they can fly. In the first issue we see them talking to Gateway. They then figure he came here to tell them Marius, their brother, is back. They go to the airport to get Chamber when he is attacked by Emplate. The M-Twins go to attack him but is held telepathically by Emma Frost. M-Twins didn't like it that much and yelled at Emma never to try that again.
Upon finding Hollow on the school grounds Monet explains to Emma and Banshee how many similarities Hollow and Emplate have. She then goes on to say Emplate must of been feeding on Hollow. Monet goes with Emma to find Hollow she knocks down trees to make it easier for Emma but Emma doesn't appreciate it so Monet flies off. She then comes back and save Emma and tells her she doesn't like how Emma goes into peoples heads and what she wear.
When Jubilee, Skin, Synch, Banshee and Monet went to Canada they ran into some problems. The security guard told them that there was a problem with a bridge there, so they would have to take a different route. Monet tells them that before they came there she studied the topography charts and that there is no bridge. They travel down the road some more to find out a mutant is holding up students in a school. Monet with the order from Banshee takes out some guards without the others noticing with the help of Skin. The Orphan Maker comes and Banshee fights it with the help of Monet. This is when Banshee figures out that Monet is actually stronger than she appears or that he thinks. Monet recognizes him from the X-Factor files, and tells it she doesn't want to hurt him. The Maker then throws Monet into a car.
When the students are allowed to go into the city Emma is kidnapped by an alternate version of Marrow and Caliban. When Generation X go to save her Monet is in a trance and cant be moved, so the rest leave her. She then comes back to save Jubilee from dying from rocks of the sewer falling on her. From Chamber's powers he accidentally blew up the girls dorms. Monet roommate was Jubilee. Jubilee woke up in the middle of the night and found Monet reading in the dark. She asks her what is she reading and Monet doesn't show her. It comes to a little tussle and Jubilee pulling too hard rips the pages and fall. Jubilee then finds out Monet was coloring. Monet then goes and talks to Gateway, asking how much longer do she need to be around these children and she then just levitates with him.
Monet goes with the rest of the group to Cassidy Keep after Cassidy had a dream about it. They are transported to a magical realm where it lacks oxygen Monet being the first to know this. She then fixes the machine that is broken but needs a energy source and Chamber powers it.
Monet goes to the party with the rest of Generation X, she goes with Synch. Monet tells Synch she is going to go and check on Sean, when she gets their she sees him passed out so she telepathically talk to Emma who is surprised not knowing Monet had telepathy. She then flies him to the medical room.
The real Monet, as some in Generation X have noted, is actually even more obnoxious than the twins when they were impersonating Monet. Thus, the Monet that the twins became had a personality and powers that differed from their own as well as that of the real Monet.
When any siblings are merged using the gestalt ability, they become privy to each other's deepest secrets and fears; this is attested to by Marius after a gestalt in which the twins and Emplate became an entity called M-Plate. In fact, it's how the twins learned that Marius killed their mother, a chilling fact he later revealed to Monet and their father.
When the M Twins were masquerading as their older sister M, they called the mysterious mutant Gateway mentor. It's likely that Gateway knew that this Monet was actually the St. Croix twins. When last mentioned, the twins were returning to live with their father in Monaco, Algeria. It's unknown how, if at all, they've been affected by Decimation.
Earth 295 - Age of Apocalypse
In the AoA universe, the twins were known as "The Monets" and aided Emplate. They are used as the trackers following Christopher Summers. They found and started fight him, during battle Corsair shot Emplate and killed him. The twins fell in a catatonic state so they become useless for Sinester´s plans.
It seems that they have the same powers in this reality. They complete each one phrases.
X2: X-Men United
Both Nicole and Claudette St. Croix's names are listed in William Stryker's files on Lady Deathstrike's computer.