If your sister in law is ap ain like that
say that you want to go shopping WITH them
take them to chuckie cheese or some other kid place,
for lunch, and then go to toys r us to get them kiddy gifts!
Then you help them choose, and they don't feel the pressure of mom
and if your SIL wants to ttake it back
they already really like it and want to play with it
or have them come to your house and play with it before they go home!
then she REALLY can't take it back because you'll
obviously throw away the packaging