How can I get my SIL to like me?

My husband's sister isn't too big on me. I really don't know why.. I am a very nice, easy going person. She is the youngest in the family, but still 26 yrs old. So it's always awkward at family get togethers because she doesn't speak to me and I don't know what to say to her. She's pretty well off money wise and my husband and I aren't, she is very church involved and we're not, etc. So we don't have much in common. A few months ago, she had a miscarriage. I didn't know what to say to her (being that we've never really been the best of friends) and also, my mother had a miscarriage when I was younger and just wanted to be left alone. So I didn't say anything to her about it. Probably not the smartest thing to do, and yes I wish now I would have but I'm only human and I make mistakes. So I found out later through his other sister (who I get along great with) that she was mad I never said anything to her. So a week or so later I called her and told her I just didn't know what to say but I was sorry for her lose and call me if she ever needed to talk. She was nice and said thanks, etc. A few weeks later, we found out I'm pregnant. So we had to break that news to her which I'm sure didnt help my case at all. However, a month or so later she found out she was pregnant again and is doing well with this pregnancy. That's been a few months ago. My husband recently found emails btwn his sister and mom where his sister was saying how last time I was over there I was being "pissy". No.. I wasn't! I laughed at all the jokes and tried to put my 2 cents in on everything but once again, not much I can talk about. But I was by no means "pissy". The rest of his family likes me because I don't think I'm a horrible person.. but why doesn't she like me?! I am about to give up trying because I feel like everytime I make an effort to make conversation with her she is just short with me. If anyone has any advice I would appreciate it.