Car seat for 3year old

[robyneHet]

I'm slightly concerned that MIL has bought a Group 2 car seat for my DD who has just turned 3 in March...it is the kind which is a booster seat with back on that is basically loose in the car and held in by the seat belt that is obviously around the child. Is this safe?!
I've done a bit of googling,as you do, and it appears that group 2 is ok for 3year old if over 2.3 stone (which she is) but is it ok to be the kind that isn't a 4 point harness car seat?

It's best to stay harnessed until 18kg which is what most harnessed seats are suitable until. There are some that harness to a heavier weight too

Yea...doesn't seem very safe to be unharnessed at 3 to me?! Suppose it must be tested and deemed safe if that's the law but I'm not keen I must admit...bit awkward now when MIL has bought the seat and it was recommended to her in the shop 🙈
I have Group 1 car seat in my car which I think will fit her until she is 4?...then I'll obviously upgrade to Group 2...and 5month old will move into her Group 1 when he is 9 months! Love a plan 😜