MIL put 20 month old forward facing

So my fiancé apparently doesn’t see any issue with this because his mom “Raised six kids who all turned out fine” but I feel incredibly pissed off. Our daughter is still rear facing and we (or I guess I since I’m apparently the only one who cares about this) are planning to keep her rear facing until at least three years old providing she continues to meet her seat requirements. So his mom dropped by randomly today and decided to take our daughter to babysit for a few hours and as they were leaving I noticed they put her seat in forward facing. Naturally I questioned it and was about to go out and stop them from leaving but my fiancé did not see a problem with it at all. We had a fight and I ended up sitting alone for probably three hours crying because I was so upset. Has anyone else had a MIL not respect their rules in terms of car seats? How do you get people who don’t seem to care to understand car seat safety? I’m still feeling pretty upset but mainly because my fiancé didn’t take my side and I’m worried now that if they babysit again they’ll ignore my wishes. What should I do??