Right ..... so a stepmum who objects, quite reasonably, as an adult to being disrespected in her own home by her SD because her partner refuses to enforce the same house rules and discipline which apply to all the other children in the household is in the wrong Dadthelion ?
Maybe, in the greater scheme of things, complaining about "typical" teenage traits isn't the end of the world - though most of us would do it - but, being expected to supervise and take responsibility for another child when you are in effect powerless - because of your partner's short sighted and selfish Disney parenting - is something the OP has every right to feel fed up about. Why the heck is the SD seemingly more important than the OP's child, and why is she seemingly less deserving of the discipline which applies to the other kids ? That smacks of her DH thinking that his daughter is better than the other kids ....... but that's okay huh ?
All this "it's her dad's house, it's her other home, she should be welcome whenever" is fine - up to a point. But you can't have it both ways ..... the SD and her father don't get to cherry pick the bits of the OP's family life they want ..... either the SD is a fully integrated member of the family - or she isn't. I quite agree that in an ideal world her father's house (which is also the OP's house BTW) should be as much SD's as any other child. In which case, she should be part of the family warts and all .... that means same rules, same chores, same participation (as applies to other kids) in family activities, same having to tighten her belt if money's hard to come by, same meals etc etc etc. The OP should NOT be bloody well having to tiptoe round this girl for fear of "upsetting" her and having her strop off elsewhere - and the OP's partner should not be allowing this to happen. It is NOT unfair to insist that all the kids follow the same rules - as agreed by OP and her DH whatever those may be - and neither is it unfair for OP and her DH to discuss anything which alters their day to day routine regardless of who's being discussed.
Why the heck should OP not be "allowed" to express her opinion aboout this ? She's not asking for special favours herself - the main gist of the problem is that this girl is treated differently, that OP's place in her own family as a supposedly equal adult is belittled when SD is there because she has no say in what goes on and that she must walk on eggshells. It's not about SD being 15 - her age doesn't matter (though I accept it brings its own particular annoyances) ...... it's about the OP's DH favouring his daughter above all else - which is selfish and arrogant and is hardly in the spirit of blending is it ??!!
Dadthelion - you seem to think the OP's DH is quite entitled to lay down the law like this because it concerns his child and that trumps all other considerations. Well, if he was single, good luck to him - but he entered willingly into a relationship where his actions, even if they do concern his child, also effect other people. As a result it is courteous to discuss stuff .... and there is NEVER any justification for allowing one child to be favoured over others. I'm amazed that anyone could ever think that acceptable.
I'm sure if OP felt respected, able to express herself naturally and supported as an equal by her DH that she'd feel very differently about this whole issue.