I feel for your H but you're right he needs to stop with the questions and leave well enough alone. I'd hate for him to create a distance with YSS. It sucks to be placed in the middle and I can imagine just how uncomfortable this all must be for him. I hope your H gets the hint eventually and stops. 

DH asks YSS about OSS because OSS was bad about communication while in HS and now that he's off at college DH never hears anything, not even a response to text messages. I have no idea if OSS doesn't want to be at our house or if he's just lazy. I lean toward the latter because that's his personality in general with family type things. But it's not YSSs job to keep track of OSS or to inform DH of what's up with OSS. Sometimes YSS knows even less than what DS knows (DS and OSS keep in touch and sometimes DS tells me stuff).

I mentioned to DH once when we were talking about how uncomfortable he is talking to his dad/how uncomfortable his dad is talking to him, that it's not "just the way we are" and that if he wants a better relationship with OSS he, DH, can make it happen. (But going through YSS isn't the way. I wonder what will happen in two years when YSS is gone. DH will know zero about his kids.)

This is tough.  DH does this with YSD sometimes.  Sometimes it's a legit question asking if she remembered to give OSD a card.  But then follows up with "Did she like it?"  YSD usually just shrugs or says somthing non-comittal.  She either doesn't know or doesn't want to tell him what the reaction was - she gets squirmy when she's fibbing or not coming out with everything.

YSD also is NOT talkative about things that mean something, which is a problem; drives DH and me NUTS. She just clams up around us if we need to discuss important things but we KNOW she's discussing it at BMs. We also know that OUR household is discussed at BMs but it's super not ok to discuss BMs household at our home. 

I am glad to see that DH isn't holding back much like he used to and is calling her out appropriately lately. 

Again, PAS - BM is gloriously right and wonderful and DH is bad and to be avoided. 

I think all kids get uncomfortable when you start asking too many questions, it feels prying and invasive. I'm sure if your DH was patient and listened he'd find out about the two older kids in an organic way. Some people just lack tact and subtlety.

We rarely ask questions about BM's house. Even seemingly inconsequential questions, like asking how their dog is, get us nasty comments from SS. I feel for DH because he has no insight into the lives of his children. Once, one SS talked about something at BM's house and the other one shouted at him, "DON'T TALK ABOUT WHAT GOES ON AT HOME!!"

Meanwhile, BM gets a complete re-cap of everything that happens at our house. 

I really despise mothers who use their kids as pawn in this way. Especially when those same mothers are always accusing others of "putting the children in the middle."

I honestly don't really see how it's putting YSS in the middle.. asking low key questions about his brothers.  If he was asking YSS to communicate with them on his behalf.. THAT is putting him in the middle in a more direct way.

It may be that he just doesn't like getting grilled period.  I kknow my SD's would get short when you would pepper them with the 20 questions about their day..  communication is a delicate thing... I think you should encourage your DH to back off just a little.  How was your week?  Your brothers doing ok?  and leave it at that.. not pry so much about details unless offered.