I adopted 2 spayed female siblings from our local shelter 3 weeks this coming Friday (Nov 21) who are 10 weeks old. My question: Why is one kitten trying to nurse off her sister? I've read about situations where kittens 10 weeks and older nurse from the mother cat and mother knows best but my situation is different where one sibling is trying to nurse from another.:confused:
Both kittens have been eating a high quality wet food 4 times/day supplemented with an equal high quality dry at 1/2 Cup/day. Since the first time they were in our house I have increased the wet food by a total of 0.40 oz/day.
Any help is greatly appreciated.

Possibly the kittens were weaned too early, and some cats do what's called "wool sucking" they nurse on things like blankets or people's tshirts because it calms them and makes them feel better - our cat did it her entire 18 years, she was weaned at only 5 weeks which is far too early, and every cat is an individual so weaning at the normal 6 or 8 weeks might still be 'too early' for some. You might try wetting the dry food with a little water 30 minutes before you feed the kittens and see if that helps.

Otherwise, don't try to discourage the behavior, it's normal and probably making her feel better and so long as it doesn't cause fights it's fine - if it does start to cause fights then distract the kittens with a bell toy rolled across the floor or otherwise invite them to play or distract them in a non-harsh and confrontational way because cats are pretty sensitive to negative stuff and you don't want to make it worse.

I see now that you are feeding wet food as well, that's great - I think it's probably something that'll go away with time. Whenever I've been around pups that are allowed to nurse until the mother weans them and continue to stay with mom they still do "milk checks" like when they were nursing - they check all the teats and go back to the puppy behavior I've seen dogs do it all the way up to 8 months old. With time the checks become briefer and less frequent... I still say it's just a comfort thing, nothing to be concerned over. I've also seen a 4 month old puppy who was well weaned nurse on a cat who had nearly weaned kittens, and puppies almost weaned do "milk checks" on aunts, siblings, and other female dogs who aren't even nursing and have never had a litter.