Is one allowed to go to shabbos sheva brooches in year of aveilus for mother ? (Invited by good friend who’s daughter getting married )
In general it is not permitted, because it is a seuda of simcha, which may not be attended even on shabbos. There is an exception however for a sibling to attend if it will look obvious that they are not attending, but it doesn’t sound like that in this case, because very often even good friends are not invited for sheva brachos.
Pnei Baruch 20-16, Divrei Sofriim- Aveilus 41-23 & 39.