What exactly constitutes a nervous breakdown?

"Nervous breakdown" is a phrase that people use a lot, but it's not really a clinical term, and it means different things to different people.

I'm sorry about your brother. I think the only thing you can really do is tell him how concerned you are and encourage him to get counseling. Since he's an adult though, it's really on him to initiate any help (unless he or someone were in danger).

It's hard to say whether the timing of these issues is related to what your mother did, but it's possible. Only a therapist that really had time to understand the whole situation would have a chance at really understanding that dynamic.

I don't know but it sounds like your heart and your gut have a good idea of what might be going on with him....I don't really know what you can do besides call any other family members that might be in the area. Though if he's going through some sort of a mental crisis I would think that he is probably not going to be open to conversation about 'what is going on', kwim? This is personally distressing to me because my mother went 'on vacation' and left her whole life and 5 children behind (though all but 1 of us were grown and did not live at home or even in that town anymore)--we have not heard from her since 2001 (wow, I can't believe it's been that long. fyi before anybody worries she's been abducted, etc. she's not a 'missing person' because she IS with a family member, that's a whole other story), I would hate to think this might seem like a 'solution' to any of my siblings when we are her age. Anyhow, I'm hoping things turn out alright for your brother and his family (including you).
The only other thing I can think of is can you invite him to come stay with you for a week or so? Maybe it'll let you see if he's being rational or is having serious mental health issues. If he does come, I wouldn't try to talk with him about anything necessarily, just be open and available and supportive of/for him. Good luck.
(I reread your post and I do NOT want this to come across snarky AT ALL, but remember that it is your brother's life and while you obviously want the best for him and don't want to see him make a big mess of everything, all you can truly do is be a supportive bystander. Whatever pain he is going through with all of this change is surely weighing heavily on him. The only thing he can do is work his way right on through it--the best thing you could do (IMO) is be a supportive sister and listen if he needs it)