One kind of assessment that I've seen recommended repeatedly to parents of kids with behavioral issues is a complete neuropsychological evaluation. A neuropsych evaluation will be able to pinpoint where a child needs help, what level the child is functioning at in various areas. We haven't done this, yet, because of the expense (it wasn't covered by insurance when dd was in therapy)--we learned enough in two regular psychological evaluations to help dd, and couldn't really justify the expense. Our psychological evaluations consisted of interviews with us, a parent questionnaire designed to score dd's functioning in several areas, information from teachers, and a session between dd and the psychologist (including some questionnaires dd answered that scored her in the areas of anxiety and mood). Also, what kind of evaluations have you had done so far? If you go to conductdisorders.com, the parents there are a wealth of information about types of assessments and the kinds of things that go along with ODD.
We found that in addition to the two psychological evaluations, family therapy (me, dh and the psychologist-no dd) really helped us further pinpoint dd's issues--lagging skills isn't something that necessarily has a formal diagnosis, and not something that's necessarily going to be seen in the psychologist's office. Parenting courses and workshops just can only help so much, and aren't tailored to my individual child the way family therapy is. The family therapy was excellent, because we could sit down weekly and say this is what happened, this is what we did, this is how it worked out...and through this process we could figure out things like "dd needs to learn to regulate her emotions" and "dd needs to learn words for her emotions" and so on. Sometimes it just helps (with regard to figuring out what's going on with your child and how to best address it) to talk over the behavior and surrounding circumstances with someone. (And yes, we did have to examine our behavior as parents and face the idea that some of the things we were doing were potentially contributing to the problem in some way. Not easy, but sometimes you need that objective person to help you see these things, so you can more effectively help your child.) I specifically sat down with our therapist and told her we wanted to take a proactive, skills-teaching approach to addressing dd's behavior. I outlined examples of skills I thought dd needed, explained my discomfort with removal of points. She was totally respectful and really helped us find develop that proactive, skills-based approach. I think that this kind of approach is still not all that widespread. Our therapist had read The Explosive Child but was still following a rewards/punishment based program designed for ODD until we clarified what we wanted. I think that if she had been really married to the rewards chart idea and reluctant to help us do the proactive, skills-teaching thing we would have found someone else to work with.
Figuring out how to teach skills (not just identify what skills needed teaching) is another thing I loved about family therapy. I'd be all out of ideas, and I'd get to therapy and say "this is what's going on, I don't know how to help with this," and the therapist would have great ideas. So for instance when it came to calming down/staying calm and communicating her feelings, I had tried so many things. Then the therapist suggested dd come in for a session on feelings, just an educational thing about feelings and how our bodies feel when we experience certain emotions. Then for home, at the therapist's suggestion, we started using these feelings charts but at first
only-and this I never would have thought of-
when she was feeling good. Talking about feelings when she was upset was both difficult for her (too aroused to learn and think clearly) and a trigger to make her more upset (because this had been a hot button in our family for so long). Another great thing our therapist suggested was creating opportunities for dd to be flexible with her siblings, setting her up for it and reinforcing her when she did (but not verbally, the therapist picked up on the fact that for dd that can be embarrassing or too much pressure so we ended up with a fist-bump thing that dd really responded to).
Personally, for the touching sister's things I would probably make the target "ask for, and get, permission before using someone else's things." Also,
why is she annoying her sister intentionally? (this is something I would ask myself.) Yes, that's a hallmark of ODD but also, it's likely that there's some kind of reasoning behind it. Most obvious, she might want to use her sister's things--and she's grabbing because she's impulsive, or thinks if she asks the answer will be 'no' and she has difficulty coping with that (an inflexible kid, like mine, wants what she wants and has difficulty accepting anything else--so flexibility is a key skill here). Impulse control and flexibilty are skills she can learn which may lead to less grabbing. I also know my dd annoys her siblings more when she's feeling anxious, bored, or lonely. If we address teaching the skills to cope with those things, she becomes less likely to annoy her siblings.
We did the timeouts for hurtful behavior with an emphasis on calming down. Dd got aggressive when upset (and she could go from happy as can be to raging mad in a heartbeat). So we'd say "you're too upset to be safe. Sit here until you're calm, then we can talk." We are frequently emphasizing calming down and staying calm around here. That's really the focus and function of time out here, it's time to calm down--it's a skills-teaching moment (even though it undoubtedly feels like punishment to dd). Along with this, we worked with dd to figure out activities that help her feel calm that she could use anytime she felt tense, anxious, angry.
I think when you're deciding what things to target first, you have to look at what behavior is the most problematic, the most urgent. For me, that's aggression toward others hands down. I like to look at it this way because targeting one specific behavior at a time, and the skills needed for that one specific goal, is easier than trying to teach all my child's lagging skills at once. It's just more manageable. Then you think about what skills your particular child needs to improve in order to function without becoming aggressive. What are the triggers for that behavior? What skills would she need to experience those triggers and respond without aggression? Once you identify those skills, the hard part is figuring out how to help your child learn them. It isn't always easy. For my dd, it was like putting together a jigsaw puzzle-there's the sleep piece, and the stress piece, and the anxiety/coping skills piece, the emotion regulation piece, the communication piece, the trust piece, the flexibility piece, etc. But you do it one piece at a time. It's slow. But I did find that in addition to making progress with that one behavior, as dd learned those skills they carried over to other areas and other things improved as well.
It's not an easy road. But there is hope!

Remember to take care of yourself too.