Maybe I'm interfering... but I don't think so.
Hardly! She's your daughter.
It was gently suggested to me last year that I should make an effort to speak my mind more often; my views are no less valid than anyone's.
Amen, sister. Especially where your daughter is concerned. She doesn't need two mothers. She needs her mother and her father.
Your daughter is lucky that her dad's GF cares about her...but...the GF could use some boundaries. Don't ever feel guilty for "getting involved" where your daughter is concerned.