I think I am quite lucky in the fact that me and DH are the only ones that have children and it'll be that way for a few more years I think (even thou DH is the youngest) so I don't have the family to tell me how to do things. I do have a FIL that thinks that HE knows how to raise Andrew even thou he did none of the raising of his 4 children. He trys to tell me how to do things and I hit the personal spot and say "yes and YOU did a wonderful job of parenting YOUR 4 children (NOT)". It shuts him up fast (I don't think he was all too pleased that his youngest child married a very strong willed and headed woman
). Even today he was trying to make Andrew eat his sandwich and I sat there in so much pain (horrible baby inside) telling him that children that are forced to eat develop eating disorders (I know I was probably talking out of my ar$e, but what does he know), and I would really like it if you didn't give Andrew one. Good think for me is the MIL will always back me up on the eating thing as she never made her children eat if they weren't hungry.
I know by now I would have given your SIL an ear full and told her where to go and to get over her little hang ups she has. I had a friend that would do the comparing thing all the time and it drove us apart as I never wanted to be around her. Andrew got sick it was my fault, but if her little girl got a chest infection (as she did quite often as a baby) it had nothing to do with the fact that both the parents smoked inside the house and no ventilation and she was made to be in the room with the smoke (as a baby she couldn't move) and therefore not her fault. She would compare development with Andrew (Andrew is 3 months older) and anything Andrew was doing milestone wise her girl had to be doing it as well at the same time as Andrew. The best was Andrew crawling at 5 months and her 2 month old was as well YEAH RIGHT.
I wish you all the luck when you do tell her what a pain she is.