The maternal aunt has a high status in Islam, which is similar to that of the mother. You should show kindness, honour, and uphold ties of kinship with your maternal aunt.

Conclusion: your brother may be your mahram for the purpose of travel if he is trustworthy. We ask Allah, may He be exalted, to guide him. And Allah knows best.

The husband’s brothers and uncles (maternal or paternal) are not Mahrams to the wife. Rather the only ones who are her Mahrams are the husband’s father and grandfather, and the husband’s son and grandsons.

The husband’s grandfather, whether he is the father of his father or his mother, is a Mahram. So there is no blame on you for uncovering your face and hands and so on in front of him.

There is nothing wrong with a woman taking off her Hijab in front of her mother’s maternal or paternal uncles; she may shake hands with them and be alone with them, because they are Mahrams to her.