They need a will, pure and simple.
It basically boils down to whole dies first. If your Mother died first (without a will), her daughters and you will be entitled to her estate as next of kin (and your father wouldn't be entitled to anything).
If your father dies first, you would be sole-heir to his entire estate (or the portion of it he owned with your mother).
Yup and if they do get married and still don't get a will combined estate then belongs to the surviving partner and after they pass OP either gets 25% of the total estate or 100% of it depending on whether his father survives or not. (Minus taxes etc..)
I don't understand why people don't do basic things like getting married and writing wills etc.. if they're old enough to have adult children.
You hear some weird "vibes" based objections from some re: not believing in marriage but these days you can call it a civil partnership even. Also, a will seems like an obvious thing if you've got children, Really the OP and his half-siblings ought to be proactive in pushing the parents to deal with this. (Unless he doesn't get on with his sisters as he stands to gain more if there is no will).
This happened in my family with an uncle/great uncle - he and his wife had no kids and neither had a will in the end, it just happened that his wife died first and so when he passed their house and assets got split among his relatives.