You’re absolutely right I know! X
For what it’s worth, bereavement makes us all a bit doolally, and it’s absolutely understandable that questions such as yours will pass through your brain as part of processing your loss.
Ultimately, you are best served by setting those thoughts free and filing it under ‘stuff I can’t influence’.
Don’t let it be a wedge between you and your half sister, and stay in touch with stepmum, if you want to, for the relationship you have, not the potential pot of inheritance at the end of the rainbow.
For what it’s worth, we’re a blended family and I own my house outright (asset going into the marriage). I will not be leaving a share to my stepdaughter (her dad massively facilitated the purchase of her mum’s house and she’s an only child) BUT... we have taken out substantial life insurance, including policies that solely benefit her and her mum (and my husband and my wills also provide the equivalent of current child support plus inflation to her mum from the time of DH’s death until DsD 21st birthday)
So, no, she won’t inherit a share of my family home (which could force a sale my children didn’t want/cause unnecessary drama) but we have arranged alternatives that ensure she will receive a similar total value, plus whatever her mum leaves her (and her mum will also inherit enough to pay off her own relatively small mortgage, if DsD is still a minor).
Blended families are a messy situation and very variable so it’s well worth stepparents sorting all this stuff out in advance via legal channels and taking away the uncertainty that you currently feel (we differ to yours in that my husband and I both had children from previous relationships but none together, and we married at an age when life insurance and wills were already on our minds, so all this was discussed before we even booked the wedding).
Nothing can be done after the fact, so do try and let things go, and prioritise remembering the positive stuff about your late dad and your extended family.
I’m sorry for your loss 