In Part 1 of this question, we talked about the possible reasons for why your sister-in-law may be hard on her husband. In Part 2 we talk about whether you should you get involved.
Frankly at this point you might be saying, "Hell no! It's WAY too complicated." But if you're still game to consider it - read on:
So here is the checklist for being a good support:
_____ You have a decent relationship with the other parties (Check)
_____ You want to take on this role (Check?)
_____ You have the ability to use both empathy and challenge - at the right times (Check?)
_____ You have good boundaries and understand that you DO NOT have the power to
change anyone but yourself. (With the right skills a person has a chance at influencing others. But it is always up to the other to change or not.) Check?
Where to get involved? There are different people you can get involved with - SIL, BIL, Wife, Mom, Nieces. The right person for supporting one may not be the best person to support another. So you may want to think about just where you want to put your energy.
Of course, supporting your wife by empathizing and possibly challenging as well is your first priority. Supporting your Mom-in-law may also be possible, depending on your relationship. And helping your nieces feel loved and welcome is of no small value.
You could also ask BIL if he would like to talk and be directly supportive to him in that way. You say that he doesn't understand her behavior. Perhaps sharing this answer with him (and possibly Lonely Stay-At-Home Mom) might be a good place to start. Of course he will likely need more skills as well since passiveness and aggressiveness are usually found in pairs. But that is for another day...
And finally, talking directly to SIL.
First let's get this out of the way: Telling your sister-in-law to give BIL a break or lighten up can actually work in some cases - but if you don't really care about her, it's just doing what she does back at her. This approach - usually done out of frustration - will likely damage your relationship, and it can only work if she is able to admit that she's too hard and he doesn't deserve it.
If you do care and are wondering about directly talking to SIL, then I suggest the following: First check if she would appreciate your support. And you might want to make sure BIL is ok with it.
Finding an opportunity for a real conversation - whether at a family gathering, or an invitation to coffee might be useful. If the timing seems right, you could ask permission to empathetically share your concerns. 'I see you as such a great mom and caring person but you don't seem to be happy.' Be careful not to take sides about the harshness issue. This is more about helping her find her way. We all need that sometimes. Depending on whether she is open to seeing her inner power to create her happiness or not will determine where the conversation goes from there.
I hope that this gives you some thoughts on considering your potential role. Good luck.