Dining at a Brother's House During Year of Mourning

It is forbidden, and it is considered a festive meal.

The Gemara in Moed Katan 22b states: "For all the deceased, one may enter a house of joy after thirty days; for one's father and mother, after twelve months." Rabba bar Bar Chana said: "And for the joy of friends." They challenge: "For joy and for friends, thirty days!" — It is difficult. Amimar taught thus: Rabba bar Bar Chana said: "And for the joy of friends, one may enter immediately." — But it was taught: "For joy, thirty; for friends, thirty!" — It is not difficult; one is in the case of rental, the other in the case of distress." The Rema in Shulchan Aruch, Siman 391, Seif 2, writes: "And the custom is not to eat at any feast in the world for all twelve months, if it is outside his home." The house of his brother and all houses, except for the house of his parents, father-in-law, and mother-in-law, are not considered his home, and therefore it is forbidden to eat there.