Is it permissible to help serve food at a wedding party where there is singing and entertainment, knowing that I will be in the kitchen and will not mingle with the attendees?

Playing musical instruments (ma'azif) at wedding parties is among the forbidden acts (munkarat). It is obligatory to advise your family and sister about this, and to clarify that marriage should be founded on the fear of Allah (taqwa (definition)). If they do not respond, then it is not permissible to participate in the celebration, even if participation is limited to preparing food, because that entails condoning the forbidden act. Abu Ayyub al-Ansari refrained from attending the wedding of Abdullah ibn Umar's son when he saw a forbidden act there. Therefore, a Muslim should not be lenient with forbidden acts nor condone them.