When I married my husband I was 24. He had 2 children who were 11 and almost 13 at the time (the same age as my youngest sibling - in fact my brother and stepson are just 3 months apart). Anyhow, I was scared to death. I really wasn't that much older then his kids (who did and still live with their mother) and from certain things the kids said, we gathered that their mother must have told them that it was my fault that she and their father weren't getting back together (even though she had left him a decade before I even met him and had been living with another man the whole time). My stepson was wary of me until he saw that I had no intention of getting between him and his father (they are very close), but my stepdaughter was much closer to her mother and so was very hostile towards me. Their visits followed the same pattern. They'd come every other weekend, and for the first hour or two, my stepdaughter would sulk, give me dirty looks, mutter horrible things (loudly enough for me to hear) and then she'd knock it off and was generally sweet for the rest of the visit, until a few hours before she had to leave again. And she was always terrible to me in front of her mother. I can appreciate how hard it must be to try to be loyal to your mother when your father just married someone else, but at the same time, their mother had been with another man for years - but I guess my marriage to their father decisively put an end to any speculation that her parents would get back together. So I tried to understand what she was going through, and honestly, I just tried to stay out of her way. I'd make what overtures I could, but I was pretty young myself - I had no idea what to do.
I always got along well with my stepson, but my stepdaughter persisted in hating me. When she was 15 she came to stay with us for a couple months in the summer. I left for a week to visit my family, and when I came back, I discovered that she had carved "I Hate Elky" (except my real name

) into all of the candles in my bathroom. I overheard her telling her father that he should never have married me, that we were going to be miserable and get divorced (we are still happy 7 years later) and that he shouldn't be having children with me (we have 2 children together now). Her animosity towards me hurt, and I really did try my best with her. But it is really hard, as the stepparent, to come into a situation like this. Despite the fact that my husband and his ex had been apart since the kids were very young (and despite the fact that it was his ex who cheated and left) I was treated by my stepdaughter and her mother like I was the other woman who had broken up the family. I was only 24 so it was incredibly hard to deal with a teenage girl who hated me for reasons I couldn't do anything about. Their mother didn't help either - she refused to leave the kids alone with me and would only drop them off if my husband was home, and from things my stepdaughter said, it was pretty obvious her mother was spewing crap about me at every opportunity. After the candle incident, we asked my husband's niece (who is close to my stepdaughter and gets along with me) why my stepdaughter hates me so much, and she just shrugged her shoulders and said that it was nothing specific, that it really had nothing to do with me, that she would have hated anyone her dad had married.
I never tried to be a mother to them, I always treated them how I imagined I would want to be treated in their position, I stayed out of the way when it came to parenting (they already had two parents) unless it was something that affected my home or children when I had my own, my parents and siblings have always treated them like grandchildren and nieces and nephews, my mother proudly claims to have a now 18 year old granddaughter (and doesn't she look great for having a granddaughter that old!) and I always made sure they knew they knew that I cared for them, and I really do love them like my own (now that my stepdaughter is older we get along much better, although I'm still a bit uncomfortable around her because I'm never sure how she feels about me).
Anyhow, I probably could have done things differently, or better, but really, for a young new wife (who moved a thousand miles from all my friends and family when I married so I was pretty lost anyways) I was terrified to be faced with 2 kids who I knew must hate me - and sometimes it seemed like nothing I did was right. Luckily we all get along okay now, and like I said, my stepson and I have always done okay. But you can still tell the difference in attitudes. My stepson's girlfriend will talk to me and play with my children. My stepdaughter's boyfriend refuses to even look at me unless he has to and then it is with such an ugly look I'd rather just hide than have to deal with him. My stepson comes to visit as often as he can (we live about 3 hours from them now) but my stepdaughter very rarely comes up. I'm sure my stepdaughter has issues with me that I know nothing about, and I wish we could be closer, but at the same time, I have tried for years to get along with her and at this point we are what we are and I just don't know what else to do. And frankly, I'm tired of being hated just because I married the man I loved.