Just a thought but are there any others in your system who can face down aggressors better, that is, who might have more success with keeping boundaries in place? If they're there, keep in mind that they're also you in the larger sense. If you weren't multiple, you'd already have access to their abilities, which are currently still dissociated away from you. So it's not like it's cheating or something to ask them to do what comes more naturally to them. I think practicing healthy boundaries is more important than who creates or reinforces them. I suspect there is somebody who would handle the interactions with your sister differently but only you guys can decide on that.
Around my mother's funeral, everything was so intense among the siblings and my older sister (again) was making me crazy one day and I just had to escape but couldn't figure out how! So without any planning, Quato switched in and handled it. He was so detached from the family, he ended up saying something weird upon leaving like "yeah, so take it easy and everything." We still laugh at the look she gave us but it was great!