legal...do I as my mother's only child have any more rights than my half-sister, who has a different mother?

Do you mean when she dies??? Assuming his estate is already settled and she hasn't adopted your half-siblings - you (and any other children that she has) would be the beneficiaries of HER estate. Of course she's free to leave it in her will any way she wants to. Hopefully your father considered the children from his previous relationship when he got involved in another and left his estate accordingly. But men sometimes think with their little heads instead of their big ones. That's how Cinderella ended up in her predicament. I'm sure your father believed that your mother would do the right thing so hopefully that is the case. I believe in Canada, it varies by province. It is possible that if they co-owned their home and had all of their money in joint accounts and he had her as the beneficiary on his life insurance - that everything could have gone to her with or without a will - in which case your half sibling could be very screwed. I hope for your half siblings sake that your dad had a will and that your mother is not the greedy stepmother from Cinderella. There is a distinction between what's legal and what is ethical here.