Q. Someone who lost a stepfather and out of closeness with other siblings sat shiva with them, is he permitted after shloshim to attend a wedding of a close relative?
A. The obligations of Avelus apply only to a one’s biological relatives. Therefore, an adopted offspring need not mourn his stepparent.
However, some have the minhag to follow some avelus traditions, and join other siblings and relatives during shiva out of respect and gratitude for their adopted parents. As the Talmud (Megilla 13a, teaches us that one who raises an orphan in his home, it is as if he gave birth to him. Poskim mention also the honor due to the related biological relative now in mourning. (Shulchan Aruch Y.D. 374: 4, Pischei Teshuvah 374: 3, Yalkut Yosef Y.D. 6: 8).
Horav Shlomo Miller’s Shlit’a opinion is that one, after the shloshim for a stepparent can certainly attend the wedding of a close relative or friend, as it is an important mitzva.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a