I want my sister to change her baby's name
Why don't you like their current name?
I don't think it fits the baby.
You're entitled to your opinion, but this is your sister's decision to make. It's rude to tell someone else what to do with their baby, especially over something as personal as naming them. I'm pretty sure she'd be really offended if you even brought this up.
She can't really name her kid whatever she wants. I'm going to tell her mother to get her to change it.
Everyone has the right to decide what to name their own baby, just as you will if you decide to have a child of your own. It is not up to you or your mother to try to force a name change on your sister or her child.