Haha, that's a really good question, now that u mentioned it. Here's the answer to ur 質問 that i found on: link.
■Fred and George Weasley had the Marauder's Map before Harry did, so if Harry can see Peter Pettigrew in the map, then フレッド and George seemingly ought to have noticed that Pettigrew was sleeping with Ron every night. They did, however, tell Harry that they found the map in their first year, and no longer need it as they "know it によって heart", so it is possible that they have not used it since Ron (and Scabbers) arrived at Hogwarts, または that they believed that the map made a mistake, that conclusion having been drawn によって Harry. It is also entirely possible that the twins simply had no interest in using the map to look at their brother sleeping.
this is the answer i found on the official HP wiki :)
Oh mrcullen don't be ignorant. Peter was NEVER famous before harry knew anything about him. So it's impossible for those two to know who he was. Why would they care with so many students around. Are あなた sure あなた read the book? And I don't think peter sleep in ron's bed. He was a rat, he might put scabber in otherplace specifically for rat/pet. Is this clear enough? Quite an Interesting 質問 ;)
1: Ron had only been there for three years. によって the time he got to school the Twins would no longer have needed it so they gave it to someone who did. Plus, Why would they pay attention to that area of the school?