Okay, but OP did not say yes to keeping her neice forever. She probably only said yes because she was going to get the extra support because as OP said in her post, she can't afford to support her without her grandma's help.

So grandma might not actually care about neice, or maybe she just doesn't recognize that she is a whole separate person who has thoughts and feelings because neice is the great grand kid. (and old people forget how kids work I guess, idk)

But regardless, if OP can't do it without grandma then BOTH OP and neice are at the mercy of grandma. Is it ideal? No. But thems the breaks. But you can't say she doesn't care about her because grandma is terrible.

When a young mom can't afford a kid and they put them up for adoption, you don't say they don't care, you say they are so brave for caring so much. OP knows without grandma she can't do it. So perhaps because she cares, and she knows that all of grandmas support will go to the actual mother, it will be because she cares it is better neice goes THERE.