I just looked at the carseat again and it says you can change to the forward position after 8kg.
DD was 7910g at her 4month check-up, so she would be around the 8kg now I suppose. But I wont tell the MIL that, or she will just think shes always right. I don't think I could do harm by leaving her rear facing for now until I feel shes ready.

Its hard to take advice from someone in the family like my MIL that suggested that I give my dd solids at such a young age. So when she started about the car seat, I just freaked-out