What is the ruling on participating with a family member who is hosting a sadaqa feast on behalf of ancestors, knowing that there is nothing that leads to shirk?

If the intention of giving charity on behalf of ancestors is free from polytheism (shirk) and innovations (bid'ah), then there is no harm in it; rather, it is among the acts of devotion from which the ancestors will benefit, Allah willing. Ibn Qudamah mentioned that any act of devotion performed by a Muslim, and its reward is dedicated to a deceased Muslim, will benefit them, especially supplication (du'a), seeking forgiveness (istighfar), charity (sadaqah), and fulfilling obligations. It is preferable that the cost of the sacrifice be given as charity, due to the consensus on the virtue of absolute charity on behalf of the deceased.