Is there a way to distinguish between his sister and your sister on a linguistic level, aside from the wider context? If this was a conversation, I would likely know whose sister we were talking about, but as a single sentence, his sister, her sister and your sister would all be correct, yes?
Actually, "[a] sua irmã" means your sister or, his sister, her sister, their sister while, "a irmã dele" is another more precise way to say, "his sister". However, "a sua irmã" is not wrong for, his sister.
Meanwhile, "a tua irmã" specifically indicates, your sister over his, hers, theirs.
And, just to throw another wrinkle in, with body parts, family members, clothes, and a few other things (sometimes keys, sometimes not, sometimes cars, sometimes not) the possessive "minha" is not even needed depending on context and syntax.