I am Muslim from Algeria - living in Germany - and my wife (Muslim too) has non-Muslim parents and family. I received an invitation to a wedding in Church for a member of her family. Could we accept this invitation?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no objection to respond to a non-Muslim's invitation for food unless there is Haram food or an abominable action that a Muslim cannot change.
Also, there is no blame on you to enter a church unless some abominable or prohibited actions may occur. If one can change such an action, then one can stay with them; otherwise, one has to leave lest Allah's wrath may befall on them all.
Here, we draw your attention that it is very important to do your best to invite the family of your wife to embrace Islam as well as to explain to them virtues and excellences of Islam. In addition, it is desirable to present them some useful books in this concern.
Allah knows best.