Your car is a very safe car. I know, I have one too! Outboard is going to be very safe, and a good installation outboard is much safer than a bad installation center. That being said, if you just have the 1 child, you can always TRY the center. you can NOT use the LATCH system there, there are no anchors, so you will need to use the seatbelt. Once you have the seat, just try it....i can see it working well actually, although it will seriously impinge upon both outboard spaces.
If you want,you can send your sister here, and we can explain to her how she is..um..misguided that being in an outboard position is not safe. What on earth does she think happens when you have 2..or 3, or 4 or 5 children?
I personally have an AOE or a titan rearfacing for my 15 month old behind the driver, and a Nauti installed behind the passenger. My son is tiny like your child, he is only 28 inches and 21 pounds at 15 months, so I fully expect we will be able to RF until age 4 or so, and I am so HAPPY I will be able to keep him so safe for that long. Having tiny kids is AWESOME in the carseat world!