Did Shermie ever argue with Filbrick about his decision to kick Stan out of the house? I mean, even though he was mad at Stan for ruining the Ford's project, I doubt he was happy about the news that his seventeen-year-old brother had become homeless and living in his own car.
Yeah, in my headcanon, I think Shermie - even if he was mad at Stan for breaking Ford's project and even if he did get a biased story about what happened - he still loved Stan since they're brothers. He definitely would have thought kicking Stan out was too far, hence why I think he tried to find Stan in Stan's grifter/drifting years after he got out of the military. But then when he found out Stan was conning people, the multiple identities, fleeing from state to state, getting involved with crime, etc... well, to him, I think it might have proved that maybe Stan really was beyond redemption. Especially if Shermie is as straight-laced as I personally headcanon him as.
I think that's why no one in the family came to Stan's "funeral" except their mother. Filbrick probably said good riddance, Ford was missing in the Multiverse, etc. Caryn went because she probably understood the situation more from Stan's perspective. She probably also felt guilty as hell for letting her husband just toss Stan out, maybe thinking, "If I'd just tried to stop Filbrick, maybe Stan wouldn't have been on the streets, and he wouldn't have turned to crime, and then he wouldn't have ended up dead so young...". Like she felt she was responsible, in a way, but she couldn't have invited Stan home because Filbrick would have said no, probably.
Back to Shermie, I think he probably just... gave up on Stan. Although, there probably was always a part of him that wondered, knowing that his father had a temper and tended to blame Stan for everything, and he probably knew Ford was a tiny bit biased because Ford believed that Stan ruined his future. But then Shermie probably got swept into his own life with a wife and kids, so just... let Stan go, hoping he'd get better and come back, but no. He ended up, effectively, "dead" to the other Pines.
I mean, imagine you have a brother your father always said was a coattail-riding loser. Then, he breaks your other brother's chances at a fancy college. Okay, oops, mighta been an accident. Then you follow his trail and find out he's become a conman getting involved with drugs and crime and who knows what else. You'd probably give up and think he's a lost cause. That maybe your other brother and father are kind of right about him being just a lost cause screw-up. Maybe you even feel betrayed yourself, wondering how your "screw-up" brother could do things that horrible, wondering how you ever believed in the good in him.
It's okay, though, because after Gravity Falls, I assume the Stans and Dipper and Mabel tell him and everyone else in the family the truth. Imagine that family reunion...