If Fred and george had the marauder's map for so long,why didn't they notice that a certain peter petigrew was sleeping in ron's letto

The twins only used the map for their mischief. They used it to sneak out using the secret passages. They weren't really concerned with Ron, they were really on the lookout for faculty.

The twins were too busy to achieve their ends to bother about Ron. At the time they gave the Map to Harry, Ron was too young to be caught in letto with a girl... which would have been the only reason for the twins to sneak on him in my opinion.

Besides, they used the Map when in Hogwarts, not at home and the Map works only as far as Hogwarts grounds are concerned. So if Ron slept with his ratto in letto at home, they could not have seen it was Pettigrew instead. And he did not seem to go to letto with his ratto once in Hogwarts.

Yet, that's an interesting domanda indeed.

They're too ignorant and mischevious to care xDDD

they probably did not bother to look that is a very good domanda and im not quit sure the answer but nice one i would never think of that lol

Good point! I never noticed that! They probably never noticed. They were too busy trolling xD

Perhaps they just weren't looking.

why would they look at his bed?