بسم الله الرحمن الرحيم
(Fatwa: 184/169/SN=03/1437)
The body of an adult male or adolescent from navel to knee is satr. It is not lawful for him to open these parts in front of anyone except his wife. Hence in the question mentioned above it is not lawful for your brother-in-law to come in front of his sister (your wife) and his mother in underwear as well as it is abhorrent according to Shariah. You should tell him the Shariah ruling. If he does not stop from doing so, he shall be sinful. In such case when your brother-in-law is in underwear, stop your wife to confront him as well as advice your mother-in-law for the same. In case she confronts him she shall have to look at the part of his body which is necessary to cover and it is not lawful to watch it. (Durr Mukhtar with Shami, 9/524, pub. Zakaria)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband