My sibling is completely untreated and their BPD runs wild. (Using "they" to keep it as anonymous as possible.) Because of this, I had a very low contact type of relationship with them. Recently, I ended up blocking them on everything and also blocking their phone number because they got upset at our parents (for something completely unreasonable and ridiculous—shocker) and was not only saying how much they despise them, but stuff like they wish they could go back in time and "beat the fuck out of her" and push my mom down the stairs. They have a history of violence from when we were kids and a couple years back, they pulled a gun on their spouse. My mom doesn't usually take them seriously because she thinks they will always just get over being mad and they'll calm down and it will be fine, but I'm personally afraid one day they will snap and kill someone. Even after telling my mom what they said about wanting to beat her, she was going to see them the next day until I convinced her not to. (Side note: my mom will put up with almost anything my sibling does because she is desperate for them to not go no contact. I think it's insane, personally, but I'm not a parent.) I think my mom's unconditional love for my sibling and desperation to have a relationship with them might put her at risk one day and I don't even know how to handle that. And now I have to tell my mother that I will not be attending anything my sibling is invited to in the future because I will not put up with their shit, basically. And she's gonna be pissed.
I hate being the only one with boundaries.
Sympathizing with being the only one with boundaries. I’m sorry.
Also be careful with visiting your parents and making sure the sibling doesn't "show up." My BPD sister would pressure my mom to invite her secretly when I was coming over, so I had to deal with "surprise" visits by the BPD sister when I'd visit my parents
The same thing happened in my family. My bpd sister has always been violent towards me. When I say always, I mean from infancy up to and including adulthood. I told my parents that they would be next. They didn't listen. I took extreme measures to grey rock and dodge her constant attacks on me, until I guess it wasn't rewarding enough and she got bored of trying and failing to get my attention. Then one day my dad went out for a few hours and bpd attacked my mom, who was very frail at the time. That frightened mom and they (parents) finally kicked her out. Mom went no contact, and dad low contact (he still gives her money). Since then mom died. All the siblings and inlaws and nieces/nephews and extended family, except one person are no contact. That person is the go between for when she needs money/help. I'm sorry you're worried about your parents. You can't make them see what they don't want to acknowledge.