I Have 3 Kittens. 2 Are From The Same Litter And 1s Not. The Outsider Kitten Is 12 Weeks And The Two Sis Are 10 Weeks The 2 Sis Keeps Sucking The Older Kitten Tummy And My Older Kitten Looks Stres. What Can I Do?

There's not  a lot to understand... The two younger ones are seeing the older one as a mother figure and looking for the fulfillment and security that she represented.  Do you happen two know if the younger ones were weaned too early or forcibly weaned (say before four weeks),... That would go a long way to explaining the behavior... But either way... You've got what you've got!  I don't have enough details to give any specific insight, but if you've only had them together a short time, the older one should soon start putting a stop to it... You can help her by firmly pushing them back whenever you see them starting the behavior.. And I'd try giving them  a little kitten formula a couple times a day for a few weeks in hopes of helping them change their thinking of where their milk is going to be coming from... Another thing you many try is getting them a big stuffed kitty to serve as a mother figure.  Sometimes when kittens are forcibly weaned too soon there's just some physiological switch that doesn't turn off and they continue to have the drive to nurse. Unfortunately I don't have a solution for that, I've see cats that will continue to "nurse" on things (like the corner of a throw pillow) well into adulthood... hopefully that's not the case here, and you'll be able to help them figure out what's what... Good luck.