In Action Comics #876, Ursa uses a kryptonite knife when battling Thara.  While wielding said blade, it seems to have no effect, whatsoever, on Ursa.  However, when Chris picks up the knife toward the end of the issue, she seems to be immediately weakened by the kryptonite.  Once Chris drops the knife and begins to leave with Thara, Ursa, again, has no problem picking up the blade before pursuing Nightwing and Flamebird.  I mean I know she says it makes her a little sick just holding the knife, but when Chris uses it against her she is visibly sick and even begins to panic.  Therefore, how is it possible that when she is wielding it Ursa is virtually impervious to the kryptonite but if another is using the blade she is immediately effected by the kryptonite?

It only effects a Kryptonian if it penetrates there skin, that's why it only makes them sick a bit. She's not effected badly when Chris holds hit, shes just afraid of, 'cause it looks like hes about to kill her. Shes only slightly worn out 'cause of the battle.