Almost all Chinese families I know don’t believe the food offered for their ancestors were offered to gods, nor were they anything but a memorial ritual. If Paul could say food offered to pagan gods was free to eat, then so much more can we eat to food used simply for remembering our ancestors.
As to helping them to light incense, that is more tricky. Although it is okay for Catholics to offer incense to commemorate the dead, others offer incense with a very different intention. So if helping them to light incense would lead them to further their erroneous pagan beliefs, then we should avoid doing it. However, we can always offer to mention that yes, I can help you, but I will light it to pray for their entering in heaven speedily, and that they will pray for us before God.