As rabble-rousing as the two boys were, we are supposed to believe that they never once checked up on their little brother before they gave the map to Harry?
This question is genuinely difficult. I have heard that the answer JK gave is that the twins would not know who Peter Pettigrew was, or at least not recognize it, and , not knowing every singly person in the school, disregard it.
I personally do not like this answer for a few reasons.
Firstly, Sirius Black was a famous murderer, and it is a bit of a stretch to say they wouldn’t recognize him.
Even assuming his name is not widely known (considering that Harry in the third book only found out by eavesdropping on a private conversation between teachers, indicating it was not widely known among students), this unknown person would be in constant close proximity to their brother, leading to at least some curiosity over who he was.
So we may just have to assume that they didn’t know who he was, didn’t check up on Ron often enough to notice that Peter was always there, and that they weren’t particularly interested in checking up on him anyway, no matter what their mother told them. Also, they shared the Map, so the likelihood of looking long enough to notice something wrong is halved.