SIL took DS, 7mo, in a forward facing car seat last week. Hes 7mo but is very very small, 16lbs, way too small for a forward facing seat and its my belief she just couldnt be arsed to change the seats over. I was ill and she was taking my DS to his nans.
Im annoyed frankly and when I found out I went mad at DP (who knew he was in the FF seat) and I feel so
because their justification was that they didnt know - despite her son being 18mo himself. Its your business to know if you have a child and a car! DP is still in the dog house because I hold a grudge. 
Anyway, shes now got MIL to have a talk with me about it, as apparently "the seat was fine as it was a combination seat and you can use them either way" and I "needed to be corrected".
Im still fuming because she blatantly made a mistake but I wasnt going to say anything until MIL 'corrected' me. The weight limits are written on the seat. Shes got her own child and one on the way so should know, and now shes making out like I'M the one who is stupid. I just think if you make a mistake you should apologise, not try to wiggle out of it. ANYWAY...
AIBU to write her a brief, non agressive message just putting her right, and including a link to the correct regulations. Im not likely to see her and the thought her own new baby could be put into the FF seat worries me even more!