Co-Sponsorhip for a in-law

Hello i would like some help on how to write a letter for my wife's mother, I'm the co-sponsor for her but we dont know where to start, my wife haven't seen her mother in several years, also we have 3 kids and my mother in-law haven't seen them in years only talk the them on the phone and email pictures. I already complete everything else just need some help on this letter. Thanks in advance.