Maybe the aunt and uncle thought you guys were doing fine - yeah - it's an apartment - but you are RESPONSIBLE adults.
That Molly's parents are paying their bills only enables them. It isn't teaching them anything other than "you don't need to grow up".
It should be your husband's place to call his aunt and uncle and let them know you are interested in the place as well. It is NOT his place to bad mouth his cousin nor his aunt and uncle. he CAN say "Aunt Jane - hey we heard through the grapevine that you are going to let Molly and her husband live in your home...I thought I would let you know that if you change your mind, we are very interested in paying you rent on your home." Keep in mind, however, that renting from family can be hazardous...especially when something breaks down...gets broken...do you really want that mess?
I wouldn't be jealous or upset about this. That they didn't offer it to you first, to me? says that they see you as responsible adults. On the other hand, they don't see Molly as a responsible adult and this way - they can "control" her...maybe...just maybe.