How would the Matsu Bros react to a s/o that also comes from a big family with lots of siblings?
Well, just off the bat, the Matsus would probably appreciate the fact they had that in common with them. They’d know it was something they could talk to their S/O about and that they’d “get it”.
I think especially Osomatsu and Todomatsu would probably bring up their families around their S/O quite a lot if they knew their S/O had also come from a large family, usually to complain but also because they consider their family such a large part of their identity (even if they might be in denial about it).
So if an S/O had a large family and the Matsus knew...
Osomatsu would probably constantly be talking about his brothers to his S/O, knowing they also came from a large family, both to complain but also to just tell them random stuff they did. He’s “proud” of his brothers in a sense, even though he probably wouldn’t describe it as that himself, and gives me the vibe of someone who would want to bring them up whenever he got the chance to. Like “oh my god, you’ll NEVER believe what Choromatsu did today, oh my god it was HILARIOUS”. If his S/O happened to also be the eldest of their siblings, he’d probably even drop his ego for a bit and ask for some advice on being a better “eldest”, or lots of talk about “being eldest”.
Todomatsu is embarrassed at the idea of being associated with his brothers on the surface, but deep down he clearly isn’t sure what he’d do without them, and he’d probably talk about them almost as much as Oso to an S/O. He’d vent about them quite a bit, but in that way where it was sort of obvious he didn’t hate them? Like his complaining would slowly become him just gushing about his brothers antics. I think he’d find comfort in the fact his S/O also had a large family, especially if they were also the youngest of their siblings.
As for the other four, I think that while they’d be glad they had an S/O that understood that aspect of their life to an extent, having also come from a large family, they probably wouldn’t bring up their brothers quite as much as one would expect. Karamatsu I feel would be way too focused on his S/O to even remember shit his brothers did, at least while talking to said S/O. It just seems like something he’d sort of forget to bring up because he’d be so fixated on the partner and want to focus all his energy on them. He might talk a bit more about it though if his S/O was venting about their own family and asked him how he felt about it, but he’d probably have to be directly asked.
Choromatsu seems pretty good at having his own life outside his brothers, even better than Todomatsu. Mostly because he utilizes the otaku subculture as his other “family”, but I feel like he wouldn’t really bring up his brothers shenanigans to his S/O unless it had been something REALLY bad/annoying to him. Plus, he’s probably anxious about the idea of worrying his partner even further by revealing too much about how his brothers are like.
Ichimatsu just isn’t much of a talker in general, especially not about personal stuff like family. He’d definitely be willing to talk about family stuff though if his S/O brought it up, but I could see him seeing the time he spends with his S/O as sort of a “break” from his brothers.
Jyushimatsu probably doesn’t bring them up much mainly because he’s not really sure what reason he would have to. He also tends to talk about all sorts of things, sometimes random subjects or things he’s fixated on, so he wouldn’t be as focused on his brothers to tell his S/O about them, unless maybe it was Ichimatsu. Jyushimatsu has a habit of oversharing information about Ichi in the hopes it will help Ichi open up and be more understood.