1. The OP was asking a hypothetical question regarding a shidduch he “recently heard of”.
2. The OP stated that the brother “doesn’t go to Yeshiva or have a regular seder (the boy is 19).”
3. The OP then asked “My question is should a siblings behavior impact the making of shidduchim? And if you think it should, then why?”
So the correct answer to the OP’s question is that Chazal advise everyone in the shidduch parsha to consider the girl’s brothers; Not having a regular seder (i.e. a chavrusa; a shiur; or even learning themselves on a regular schedule) is not proper. So, yes, a siblings behavior should impact the making of shidduchim. And the reason why, as explained earlier, is that Chazal stated you can expect you’re children to share personality traits with their mother’s brothers.