I'm not a friends fan but have seen some of the episodes...but obviously not that one. If I came to a shower that had a banner like that I would be thinking "wow, they bought the wrong banner and instead of forking over a couple more dollars for a new one they slapped a 'not' on it!" I don't think a LOT of people would "get it".
Tell your sister you'd rather not have that banner. If she has already purchased it just tell her it is OK not to have any banner at all...no need to buy a new one. You can mention your DH's family and it would kind of look like a slap in the face to them...sort of a "neener neener...we are NOT having a boy!!!! So take your name and ??? it!".
BTW...I know the pressure from the hubs family to have a boy to "carry on the name". I had a girl frist and they couldn't wait until I had a boy...he is the 5th!