I guess it would depend on the size of your child seats. If it is the child seats (say up to 18kg/40lb) then I would say probably not but if you are talking booster seats then you may be able to do it.
We used to have the Snug Rider in the center position and my wife and mother-in-law would ride in back with Jr. We now have the Britax Marathon, and those 2 seats would be REAL close. Buckling / LATCHing them in would probably be more of a pain. I don't know if 2 Marathons would fit side by side or not. And your mother-in-law better not be very big.