Fletch, I totally understand re: your sister...there is only so much you can say before you are crossing the line hey.

Caro, yes the pegasus is another good alternative for you as it has the anti sup clip attached, so you can make sure your child doesn't sumbarine down.
It would be even safer with a 'H' harness thow as it has been found that the adult seat belt is not safe for children under 5 years of age due to their underdeveloped skeletal system. With a harness, the impact and force is spread out over the childs body.

I applaud you guys for taking the innitiative to look into your options.