Is it bad being too close to my stepdaughter if the mother doesn't like it?

I have the sweetest step daughter ever. She's 4 years old and she's so well behaved and cute. I've been married to my husband 2 years now and I've been in his daughter life since she was 1. My husband has got primary custody of his daughter, his ex wife wants full custody now and they're going to court over it. My step daughter sometimes calls me mommy I think she's a bit confused since she sees me more than her mother. The ex wife isn't too happy about me being to close to her daughter and has told me to back off. I love the girl very much but I don't want to replace her mother. Is it bad being to close to the girl?