You definitely have a right to feel how you feel.

I couldn't be in your situation myself, I also need lots of alone time and calm.

I think your bf needs to make you his priority in this situation. He needs to care about what you think and feel.

His niece needs help. Your bf can find out about social services available for her and her immediate family.

You are being treated like an unpaid babysitter with no rights, that is not fair.

I know from experience how hard it can be to figure out how to move while stressed out, but that might be a good option for you. Are you in therapy to help you during this time? Is there any case management available for you?

Big hugs to you, I sure can relate to your situation.