Hey Mamabear... I understand how frustrating and worrying it must be to try to explain such things to young children. They can't really grasp everything we want them too.
The thought that came to mind when reading your post was just stay positive with them and let them know that just because baby C is going to have a different mommy and daddy, she's still their sister. Then when M says something like "why are THEY buying presents for MY sister" you can respond with something like "Yes C is your sister, but *adoptive parents* are going to be C's mommy and daddy and so they're buying her gifts just like I buy things for you" then you could also explain how you love being a mommy so much you wanted to let them be a mommy and daddy... and so you are C's "tummy mommy" and they're going to be her regular mommy and daddy.
Yes? No? Maybe? I just think that's a fairly simple and child appropriate way to explain things. I would also try to find ways to involve them in the adoption so they feel like they're "acting" instead of being "acted upon" (I would imagine feelings of being acted upon would lead to feelings of "is mom going to give ME away too?)
Good luck!!