She's single btw, and although she's good with both dd's, she's often sarcastic to me about all sorts of things!
For instance, over Xmas we were all having a light hearted convo about snoring, both of my girls snore and we often threaten to tape record the noise as they don't believe us! My SIL asked if her brother, (my dh) does and I said no, as he honestly doesn't! My SIL got in a huff, "Oh for god's sake ONTHEPIER, why don't you just join in and say yes, there's no harm in it is there?" She looked at everybody, shaking her head which made me feel quite awkward.
Our youngest has just started school and is quite shy. If she's invited to a party for instance she always wants me to stay, which I don't mind at all. I get a buzz out of seeing her enjoy herself, and I can chat to the other mums and help out if needed. My SIL just groans, "For goodness sake, you're not still STAYING at these things are you?" as if it's the most boring thing on the planet, it's not!
We recently had a weekend away and she couldn't really understand why we wanted to take the kids with us, she kept offering (in a pushy way) to look after them while we just went and had our "own time", and didn't take kindly to the fact that we were looking forward to a "family" weekend, just the four of us. Also when my youngest was breastfeeding my SIL used to get frustrated when we were all out together and I had to stop regularly to breastfeed, she asked why I couldn't "knock it on the head" so outings weren't so regimented. When I said I was happy to breastfeed as long as she needed it, good for the baby etc, she looked at me as though I was from a different planet!
If I talk about having enjoyed an outing to a playpark or somewhere with the girls she'll make a comment, "Oh you really know how to enjoy yourself don't you!" My dh and I do enjoy our own time as well and have friends, but I really enjoy my time with the children too, is that a crime?!
I wonder if it could be jealousy although my dh just thinks his sister is a totally different type to me and it's just her way. He can't understand that I feel a bit bullied and nervous around her!