This sounds very familiar. Our 8 year old always had a very hard time dealing with particular kinds of situations - territoriality, sharing, letting go of things (but I may use it for arts and crafts!), etc.
Our approach revolves around several principles:
Acknowledge and validate her feelings: yes it can be upsetting when siblings touch your stuff, yes it can feel great to get gifts, yes it can feel wonderful to have all of your stuffed animals in bed with you. This lets her know that she is accepted, that she has a partner, that she does not need to fight for her feelings.
When appropriate, get her to compromise in any way, some way, however small, e.g. stuffed animals are okay but no poky dolls in bed, maximum of 10 soft animals in bed, etc. Some compromises can seem ridiculous... 10 animals in bed, huh??? Just remember the goal is to slowly and positively teach her how to work through these feelings.
Be firm when needed, but in a positive, cooperative, understanding way - yes you told your brother not to touch your stuff, and yes it feels very bad when he touches it without asking - but we just cannot have people hitting other people, okay? I understand that this is your favorite doll and you really want to sleep with it. I really want you to spend as much time with it as you can... I am really sorry but I just cannot let you have any poky toys in bed because it is dangerous... etc. Work with her. Stay calm, speak in a positive, respectful tone regardless of how strange and maladaptive her behavior may seem. Give hugs. Remember that she is experiencing genuinely strong feelings.
Try to secure cooperation of the siblings. Ask them to work with you to help her.
Use humor whenever possible, e.g. make things absurd - offer to bring all the toys into the bed, do it, laugh together, then work towards a more reasonable situation.
Look for creative solutions to replace conflicts. For example, our morning clothes selection got easier when we pre-packaged clothes in gallon ziploc bags the night before.
Use music/songs to diffuse situations, use distractions. Kids may place extreme value on the object of their attention. Adults may try to reduce that extreme value. Instead, try to shift the kids' attention.
Aim for small victories, and savor them when you get them.
Remember - this is not a "personality flaw"... The crocodile part of the brain is stronger in some kids than others...