Do the mourning laws of a Yahrzeit apply to other relatives of the deceased?[1]
No. It only applies to the children of the deceased and not to his siblings or parents. Thus, a parent or sibling may go to a wedding on the night of the Yahrzeit of their child or sibling.
[1] Pashut, as the Rama and all Poskim only mention the Yahrzeit of a parent; Nitei Gavriel 22:1