Can you ever reunite kids separated in a custody battle?

Of course, yes. Every family is different but relationships can always be strengthened and renewed. Furthermore, therapy may not be necessary depending on whether parent(s) and kids are able to communicate clearly and effectively. People, especially kids, are very resilient. It will not always be easy but it will be worth it.
Yes! but carefully. Complex custody situations may separate a child from a parent or sibling, and this estrangement can cause pain & harm in the family for generations. But people in such families can be very sensitive and each have their own wounds, so it is best to go slow & cautious, ideally working with a phd-level family therapist with lots of experience with reunification and estrangement. Don't give up!