Simply put; ask your sister in law if she NEEDS it back or even wants it back. Is it your mother in laws bed??? Where is the 2mo old sleeping?? What "deal" was brokered when you received the bed? Is the bed an heirloom of some sort?
Sorry for the additional questions but, personally I feel it is between you and your sister in law to work it out. Be kind, honest and sincere; explain how you feel about rushing your daughter and not wanting to change beds just yet. Offer to buy them a new crib...if they get offended, well, that's just plain silly!
However, if you feel it will keep the family peace to give the bed back because your S.I.L wants/needs it; you will just have to explain to your daughter that her cousin needs a place to sleep, too... and make her a part of getting new bedding for the big girl bed (or something...thought about the safety bed rails??) and make it exciting...it's hard, I know. At the very least(and this may sound harsh, I'm sorry), she WILL survive this :)
I just had an idea! If you HAVE to give the bed back in a month, maybe making a calendar or one of those paper chains to mark the days off until she gets to have her own big girl bed would help! ??? Just a thought.
Good luck!