The bassinets have strap cover things over them, they are as safe as the seat you are sitting in.
I wouldn't take your car seat on the plane, it wouldn't fit well and is a PITA.

Your capsule will come out of your car, anchored or not, the bolt remains in the car and there should be a clip thing that comes on and off.
Car seats here have a place so the sash seat belt goes through the back of them, as well as an actual strap to the anchor. So they are strapped in 2 different ways. You can use them without the anchor point if necessary.

I'm not exactly sure what you are meaning WRT to the sister's seat, but you will need a reversible car seat to do what I think you mean - which is be able to strap it in the car via the sash seat belt and not a bolt anchor point.
Your capsule will not anchor anywhere without the base unit, a car seat does not have a base unit.

Also, your capsule will not fit on an aeroplane seat.


HTH