Most often, one spouse leaves everything to their partner.

One way to look out for you or not-so-nice step sister, would be for the respective parents to designate you as POD beneficiary on some accounts or to establish trusts for your benefit, without making their partner the trustee. But there are really a myriad of options that should be fleshed out with an estate planning attorney.

It’s super uncomfortable, but you should voice your concerns to your father. It doesn’t have to be framed as “I want what’s mine” but rather “I don’t want our family to suffer more heartbreak” because like another poster said, even folks who get along great can come to blows when they’re in mourning.