Hi A., this is a tough spot and all i can offer is to talk to your husband and make your home by your rules!
meaning yes, you need to step in and put your foot down with her. children need to have disipline by both adults. and you are disipline your two sons when they get into trouble right?
and you are not disipline her?
why she must feel different that you love those boys enough to do something and then you don't treat her the same way can be very confussing to her. (i know you love her)
plus in the long run your boys will pick up her ways if you continue letting her get away with only having to face her dad.
i too was in a home with step parents on both sides. i was not treated any different than if they were my birth parents. i learned real fast to show them the out most respect because they lovede me because they wanted to.
have you tryed to spend girl time with her alone? maybe she is feeling she is shareing her dad and step mom to two new members and she wants to have one or both of you own every now and again.
remember the new ness of these two babys maybe wearing down and for her to get attention she must act out.
these are just ways to look at it. i am not saying that any of these things are the reason but think about them.
i wish you luck and keep a open mind.
oh yeah another thing to look for is does she think by gettting along with you she is betraying her mom.
i know at that age is when i felt i need to pick sides and for a small period of time i wanted my dad to me and only me!
she is really trying to say something and you guys need to find out what.
again good luck,
T.