Her hoarding is not your problem. She is a grown adult.

Sell her your share.

I understand that you might feel like selling her your share of the house is enabling her. But the reality is she's moved in and already began cluttering up the house so there's really not much you can do at this point. The horses out of the barn.

If you can afford to, I would set money aside knowing that you may need it for interventions, therapy or cleaning services, but I think that all of those need to be decisions made in tandem with your nephews. Has other people have winded out? Hoarding isn't extremely hard disease to treat and she needs to be motivated to change her life.

Hoarders cannot be fixed, OP.

If you don't want her to live in your mother's house, you'll have to buy it from her.