J.,
I feel for you. I am a stepmother of 2 children (14 and 9) from 2 different homes, plus my husband and I have a 4-year-old together. I really don't have a relationship with either mother. My stepdaughter's mother stopped speaking to me 7 years ago, because she stated I offended her by combing her daughter's hair during a visit with her dad and I. For years, my stepdaughter felt like she was betraying her mother when she had fun at our home. She is finally moving past this.
My stepson's mother says hello, but there has been constant tension since my stepson came to live with his dad and I 5 years ago do to behavior and school issues. We stress good behavior, good grades, and Christian values to our kids. My stepson mother let him do whatever he wants when he is in her home. It is an ongoing struggle to keep him on the right path.
My relationship with both kids was good until my 4-year old son was born. My son was really cute and chubby as a baby which drew a lot of attention to him when we were in public. This really got to my stepchildren. They were very jealous of the attention that he received from others and from me. They are beginning to like him, but still at times band against him and don't want him around, which really upsets me as a mother, because he really loves them and misses them when they are not around, but I have to back off and not react.
I said all that to say this. Do your best. Let the kids know that you care by what you say and by spending time with them. I have learned that buying them things does not buy them love. I do not say anything negative about their mothers. Blended families are a lot of work and just when you think everything is okay, another issue arises, but in the end, the children will see that you care.