I'm the sister in the middle of estranged brother and mother. A fight of10 years. Neither one will apologize .brother won't speak to me if I invite my mom. I am inviting her

That's your brother's problem not yours. If he can't put aside his differences with your mom for his niece, that is on him not you. You invite them both to share in your daughter's accomplishments and to honor her.

This is no time for family squabbles, differences, when your daughter has accomplished a milestone ! Of course invite your mom. Why would you leave her out !

Invite everyone and don't discuss their conflict. It's not about them.

I agree, invite everyone
This is brothers problem, if he does not come.