Hello and Happy New Year to all. This is my first time posting here and I am doing so because I feel I am at my wits end. I am having a severe problem with my SO and her 11 year old daughter, which is affecting my relationship with my biological children. Please allow me a minute to provide some back-story:

I met my SO five years ago, when her daughter was 6. Her father was completely out of the picture. My SO's aunt (who has no children of her own) stepped in and provided support but also spoiled the child tremendously. Aftery SO became pregnant with our first child, I sold my place and moved in with her. Two years later, we had our second child. So there are now five of us under one roof, but it feels like two separate families. And now here's where I could really use some advice:

Whenever I try to discipline the step child her mother (or aunt) do the polar opposite! Example: recently I told her she couldn't go out with her friends because she didnt do something we asked of her. But the aunt steps in and takes her to the freaking CIRCUS! My SO refuses to say anything to the aunt and tells me that if I have a problem with it to "take it up with her not me".
Well, here's the problem...my biological kids are getting older now, and they see that they don't have to do what I ask because they'll get their way anyway. Everything I read says I'm not supposed to discipline the step child, but how does that work when two other kids see that and take it as an invitation to disrespect and walk all over you?
The aunt takes my step daughter every weekend to spend the night at her house. They go skating, to movies, to theme parks, etc...and when they come in on Sunday they go on about all the fun they had, but never once included MY two.

HOW am I to feel like a father and man of the house to my two kids when they see their sister get away with murder?