Perhaps your mom meant be a surrogate in the future? Surely she couldn't have been suggesting you would give your baby to your sister!
My sister went through IVF like 10 times and had countless miscarriages. She did finally end up using a surrogate to have her 3 girls. But that surrogate was not anyone she knew prior. She never asked me bc I am 8 yrs younger and was not only very young still, but was no where even ready to think about having a baby for myself then.
Has your sister had all the recurrent loss testing? My sister's doctors missed finding the reason for her miscarriages. It wasn't until the surrogate carried a baby using my sister's egg and that baby was born with trisomy 18 and only lived a day that they realized the problem. My sister has a chromosomal abnormality that only gets expressed when she gets pregnant. The babies all have trisomy. My sister's body knows those babies can't live. But somehow, putting the fetus in another woman's body, it got past the natural defenses and was actually born. After that my sister had genetic testing done and found the answer.
I am so sorry for all your sister has been through.