I agree that your first example is a decent way of doing it. In fact, there are a few different ways to punctuate lists. A read-through of this might be helpful.
Firstly, I would use the colon at the beginning of the list (since the statement at the beginning of your list is a complete sentence).
Tom has three brothers:
If you use a colon, you'll want to break out the sister into her own sentence. (This is just an opinion, but I think it looks nicer this way and doesn't make it seem like you are shoving poor Mary onto the end of the list as an afterthought.)
Tom has three brothers: John, Edward, and Bill. He also has a sister, Mary.
I also agree with the commenter above that parentheses might be your best bet if you'd like to have them all in one sentence.
Tom has three brothers (John, Edward, and Bill), and one sister, Mary.
If we are using names which are pretty obviously gendered such as John or Mary, then you might be better off avoiding the awkward semi-colon altogether.
Tom has four siblings: