Children need to be able to get away from the conflict. They can't be constantly in the middle of it back and forth between the parents.

If you do get him to the beach, even both of them. You can get down on the sand where you are sitting on your legs and show them that exercise I shared with you. It provides the release of energy and at the same time when you let your arms down and your hands hit your legs it allows you to feel yourself. Actually, that's healthier than cutting which is basically looking for release and feeling yourself. Raise the arms up high, take a deep breath then exhale with a big loud sigh dropping arms down and against the legs and body. Do it a few times, it really releases and works.

Something you could show your brother so he can release his tension which he is collecting so he doesn't release it on the children. Ptsd produces too much cortizol and doing this exercise actually releases it instead of having no place to release or understand that's what is needed.