I cannot answer your question directly sorry, but I'd just say that you should give good consideration to whether having a joint account with siblings is a good idea. Money and family/friends is rarely a good mix and commonly leads to falling out. It's just so difficult to predict the future, who knows how people can change or their life circumstances shift. Obviously I'm saying this with zero understanding of your circumstances, it could be totally fine, but if you're on the younger side I'd just give this a second thought and consider whether there's a safer way to do what you're trying to do. If something did happen and you ended up falling out over money, you'd look back in 10/20 years with huge regret.
Just a thought...