How old is your ds?
What year is the sitter's vehicle and do all the back row seating positions have lap/shoulder belts?
If the vehicle is new enough to have LATCH, I'd get a radian 65. It installs easily in pretty much all vehicles with LATCH and your ds has 12lbs at the very least before you'd have to worry about a seat belt installation. (I'm not sure if Nissan is 40lbs or 48lbs...) It's also very narrow so that the girls in boosters should be able to buckle up beside him, and it folds for in between use so wouldn't take up much space either. Plus it has 18" top slots and a weight limit of 65 lbs.
The other option I'd consider is a cosco scenera or uptown - both of those are relatively inexpensive, narrow compared to some other seats and should install fairly easily - they just won't last him as long as the radian because they have a 40lb weight limit and shorter top slots.
He is definitely safer rf'ing at his weight - and I'm not sure about his age, so I can't say definitively if I'd push to have him rf'ing in the sitters vehicle, but if he's 3 or close to it, I'd be fine with a single trip a week ff'ing in a good seat. I know others will disagree with me, but if he is older, he is safe ff'ing - just not as safe as if he was rf'ing.
If he's less than 2yrs old, then even in the sitters car I'd push to have him rf'ing, and if the sitter doesn't typically have an adult passenger in the front seat when she has your ds, I think the Radian would be the best bet simply because of it pretty much automatically having the right recline angle so you wouldn't have to check the angle and worry about pool noodles and such every single week. :twocents: Then you would still have a high weight seat to use once he was turned ff'ing as well.