If there aren't abuse issues from the stepfather I'd hold firm. Sometimes being a good parent isn't giving your child every thing they want...in fact it very rarely is. Being able to push everyone around at 14 leads to an adult who thinks she can push anyone around and not follow the rules at 18, 22, on and on.
I've seen it myself in my friend who was allowed to run the house from the time she could talk. I was so jealous of her when we were teens...she got absolutely everything she wanted, she spoke to her mother however she wanted, she was allowed to have her boyfriend sleep over from 13, etc. Every time anyone said anything to her mother the reply was that her daughter came first.
Not so jealous these days...she's served jail time for selling meth, has gone through numerous houses and jobs and men, has two children who are being parented solely by her mother. Her brother's college fund was eaten up by her mother, who used it for my friend's legal fees. It's a mess. Her life is a mess. She has never been able to reconcile real life with her child and teenhood of getting everything she wanted, of being able to do whatever she wanted. Rules and the way normal people live their lives mean nothing to her because she is "special".
That's an extreme case of course but WHEN does it stop? If her daughter was 30 and didn't like the new partner would the mother still have to drop him? And what of the two little girls? IF there is no abuse going on, is their big sister really so selfish that she'd deprive them of their father so she can have her way in everything?
My parents didn't abuse my brother and he still left home at 15 (in the States it's supposed to be 18) because my parents gave him the option of going to school or getting a job. He wanted to do neither, so out he went.