I once read somewhere that mother-daughter is the most complicated relationship known. My own experience confirms this.
The hard facts are that this is for mother and daughter to work out, or not, between them. You have no place in this. What’s really gone on, you are not even privy to. That should tell you everything you need to know.
Further, there is nothing you can do that will not completely backfire on you. Because she is an adult, gets to set her own boundaries and you can only choose to respect them or not. I hope you’ll choose to respect them out of love and compassion.
You cannot possibly know your niece’s pain or suffering, what she’s struggling with. It would be kind to prioritize her suffering at least on equal with your sister’s. In recognizing that, it becomes clearer still that it’s between them alone to resolve in their own time and way, without interference.
I appreciate that your sister suffers, as a result of this but, in the end, you cannot push a rope. She’ll come back around, or she won’t, and it appears that there is little either of you can do about it, but live with it. So I would suggest you focus on getting your sister to recognize this. She may not come back around for years, or, possibly never. That’s harsh, but it’s the reality and you both need to grasp it.
If, in your estimation, the mother daughter relationship was healthy/solid enough to warrant reestablishing sometime in the future, then have faith that you’re right. If you’re right, then in some few months - few years, most likely she’ll come back around.
The mother should be using this time apart to own her part in things, (we all have some ownership in our ‘things’!), and reflect on how she could have been of better aid to her daughter’s needs, whatever they were, in my opinion.
In the end seeking love from someone who is reticent to give it, is a bit of a fool’s errand. Like seeking ‘true love’. You don’t seek true love, you seek to be worthy.