What should I do when I pray with my mother and she is not careful to cover her forearms and the top of her neck during prayer?

If a small amount of the light awrah (private parts) is uncovered unintentionally, the prayer is not invalidated by it according to the Hanbalis, and this is the preferred view. A small amount of awrah, whether in extent or duration, is excused, specifically that which is not considered excessive to look at, regardless of whether it is a severe or light awrah. The extent of a small amount is determined by what is conventionally considered small. As for the Malikis, they hold that the forearm and neck are part of the light awrah for a woman, and if either is uncovered during prayer, the prayer is recommended to be repeated as long as its time has not expired.