We are all someone's daughter first, then daughter-in-law.

We apparantly have more experience than you do. We know how it feels to be a daughter and a daughter-in-law. There must be a reason that for your story, most people here are on the daughter-in-law side than on the daughter's side.

That home, after all, is your brother and his wife's home. Whether she is right or wrong, it is her house.