I understand that ESTJs and ISTJs are basically busy, hard-working types who base quite a lot of their sense of self and accomplishment on getting a lot done. So, a happy and healthy ESTJ or ISTJ would probably be hard-working. (It's hard to imagine otherwise).
However, I also get the feeling that these types will throw themselves into hard work and general "busyness" specifically to avoid things like: uncomfortable emotional situations, people they're trying to ignore, deeper reflections on their lives, etc.
This is really general and I'm not sure where I'm going with it

but I was wondering if you guys could fill me in on how "healthy busy" and "unhealthy/avoidant busy" might show up in different xSTJs.
I'd say that xSTJs aren't busy with the aim of avoiding things; I think the avoiding things happens as a consequence of being busy, most of the time.
But there are a lot of instances, both with healthy and unhealthy ESTJs, when they use business to avoid things. e.g. When xSTJs are stressed, they can get caught up in their Ne, and start avoiding more important things by doing busy-work, i.e. things that still have to be done but just aren't as high priority (like what Wind-Up Rex said about stress cleaning). Also, one common xSTJ coping mechanism in the face of tough times or a personal trauma is to push the emotion away and just Deal With Everything Else. You might see an xSTJ whose family member has just died, being the one person in the family who isn't a wreck about it, because
someone has to deal with the funeral arrangements, and plan out the estate sale, and deal with everything calmly and objectively when obviously no one else is being calm or objective. (Later, their emotions might blow up out of nowhere, or if they're an ISTJ they might be able to hold it all in until they're in private)
My dad's ESTJ. From what I can tell, work is caring to him. It's tangible evidence of his responsibility, but also how he marks what's in his sphere of existence--what matters to him--and he assumes the same meaning for everyone else. He leads by example. He works his hardest to indicate how he feels things ought to be done, and takes it as a sign of disrespect if after being shown the "correct" way to do things someone choses to do otherwise.
There are two things that come to mind when thinking of the unhealthy side of work for him. The complication goes back to the fact that work equates to caring for him. Work can show up where it's not wanted as a kind of demand. It's the INFP shadow that rears it's head and gets hurt that leads to problems, "I've done for you. I shouldn't have to ask for you to do for me, too." When dad's really stressed this goes into overdrive for him. He's a stress cleaner. It's awful for everyone when dad gets into a stress clean mode and no one pitches in. It's like he put in all the hours that he ought to, then went over and above and still no one recognizes him. For ESTJs, at least, if they're working they're not avoiding, but there can still be that desperate edge. There's something that needs to be mollified.
Boy, does all of this ring true! The bolded is something that annoys the shit out of me in other people, but sometimes I slip into

and I have to force myself out of. I need to remind myself sometimes: If I want something done, I shouldn't passive-aggressively wait for someone to do it if they don't know that they're supposed to. The concept of reminding people of things exists for a reason.
Oh, it's not that I think his work is bad or anything. He seems to find it fascinating and satisfying and it is about as interesting as work can get. But it just doesn't seem well-rounded to me. I need a lot more in my life than interesting work (and my work is fairly interesting); I also need a variety of friends, spirituality, relationships or the possibility for them, etc. It doesn't seem like he has much of that, but then, it may just be that he's very private about it all. He has a lot of no-go areas, it seems.
^ I get where you're coming from, more than where he's coming from, in a lot of ways. But then again I have so many interests that I would be sad without -- and I am so intensely territorial about maintaining what little free time I have, that some people in my life used to think that I cared more about having fun than doing well in school etc

but the thing is, if I don't guard that time with my life, then I won't have it, and life would suck.
Privacy must have a lot to do with it; my dad is an INTP and he used to do the same thing, i.e. work, dinner/chatting, TV, bed, every day. But I think he gets a lot of satisfaction from time on the computer, and he finds 5w4 privacy and creative outlet in various places throughout the day, at work and at home. I wonder if that's the case with your ISTJ brother, too.