Should I leave DH alone about his family or try to get him to forgive & make up w/ them..esp for our children?

My DH has cut off all ties to his mother and only brother.
We have a 1yr old & pregnant w/another. He doesn't want either of them anywhere near our family.
FACTS:
*Family is very important to me. Unfortunately we both come from very small families. I like having a 'big happy family'.
*His brother is the life of the party. Funny. Can be charming. Nice to waiters, etc. However, he is not kind in private. Very belittling towards his wife, yelling, possible physically, definitely mentally abusive. Anger issues. When he fights, he does not fight fair...uses everything to make sure he wins.
Again, can be a very fun guy to have around.
*His mother: emotionally reserved. Very reserved. Critical. However, a very hard worker.
PROBLEM:
DH and brother have always had a very contentious relationship. He is ALWAYS right. Bro is a bully. Will keep after you until he wears you down. Both DH & mother give in to keep peace. But makes DH think that Mom never takes his side and feels really hurt...finally had 'enough'. Cut both off after brother wrote a very mean/inapprop. letter to me. And Mom basically trying to make excuses for him.
I think the fight is between me and the BIL. DH said the letter was 'the last straw'.
I know not all families are close...nor all mothers are meant to be 'mothers'. But we already have such a small family. Is it something I should keep pushing for him to resolve?


Updates
If I weren't in the situation, I would be offering the same advice.
I guess when you are in the thick of things, you think of so many "but you don't understand!" thoughts.
Right now, as DH says, we 'share' 2 grandparents (with whom we don't discuss any of this...they don't need to be brought into this drama.). Until they pass, our paths will invariably cross...especially the one that just moved in with his mother.
I hate having to find times when they won't be there so our son won't have to be subjected. And forget figuring out holidays! We are constantly having to arrange our schedule around theirs. What a pain.
I think if we didn't have those 'ties', this cutting off would be easier...you know, 'out of sight, out of mind'.
I think I would whole-heartedly let it go...but I know how much losing family saddens him. He's constantly wanting my parents to move back home (they live abroad) so our boy will have (good) grandparents.
I just get so sad thinking about it all. But honestly, not seeing them does bring less stress into my life...sad, isn't it?