We have a spare carseat. Wal-Mart sells a cosco touriva that goes to 40 lbs and can be rear facing - this is very bare bones but an extra seat doesn't need to be plush. That is what we have used for 4 yrs w/o problems.
I am almost a pro at changing out carseats but it does take work to get the seats in tight. Maybe I am a fanatic about having them really secure so I really get a workout - even with latch clips. We have britax seats and they really fit in tight plus at 40 lbs they have a strap that tethers to the back so in a crash the seat doesn't fly forward and cause a whip lash type effect. My girls will stay in the seats until 65 lbs then move to the booster. My 5 yr old still doesn't complain yet so I'm not concerned. You have quite a ways to go in 5 point harness seats so i would just pick-up an extra seat and leave it in your car. That way you know the seats are secure in case of an accident. Have your SIL do the same.