I am mum to two bereaved children. I have read the whole thread and I think I can add some things that others might not have picked up on.
Firstly, bereaved children can often access support from a hospice, even if they are not from families where a hospice was involved originally. This applies to my children and our local hospice has been amazing. Google bereavement services for children in your area and see what it comes up with. CAMHS are likely to turn your DB's daughter down and say this is normal for a grieving child. The bereavement may have occurred five years ago (and for us too), but it will affect kids forever. Even with counselling, it affects my children everyday. If you can't handle that in the future at some point, it would be best to get out now. I don't let my children be rude or threatening to others, but they absolutely have to come first if I have any chance of helping them to grow up into the kind of rounded individuals we hoped they would be before the bottom fell out of their little worlds. The fact that your DB doesn't have this same level of concern for his DD is really, really sad. I have had to give up my career (but still have a job..) as the kids worry that I am dead if I am a minute late for pick up after school. I put myself a very distant last in the pecking order as their needs are complex and changing and it is my duty and wish as their surviving parent to help them as much as I can. They do chores, they do lots of activities, but they have the security of me putting them first (whilst also meeting mine - mostly!). When you compare this to the little girl in question, as that is what she still is, it is no wonder that her behaviour is challenging.
Secondly, a bereaved child might actually want a new parent. My kids do! It must be very confusing for your DB's daughter, as she sees you parenting your own children but you are not making a relationship with her. In separated families, I can see why people say they are not going to try and replace the other parent, but in this case, where a parent has died, a step parent might expect to be involved as a parent eventually, as much as the bereaved child feels comfortable letting someone else in - all the bereaved kids I know have varying opinions on this. My kids know that no-one will replace their dad, but they would love to have a relationship with someone as their step-dad. If you don't want this at all, then I also think you should seriously consider the future of your relationship.