Being a widow, or a widower is a very specific thing. It means you were married, and you saw your marriage vows through to their ultimate conclusion.
I am sorry to say that when you lose your other half, and if your parents marriage was a Godly marriage, they the two were one made so by God. So when you lose your other half, you can be in a crowd of people that know and love you and still feel a soul crushing kind of loneliness I cannot describe. It is nothing like other sorts of lonely we all go through. The poet that said it is better to have loved and lost than to never have loved at all, forgets about the pain the tearing away of that relationship causes...
Without a doubt you have not had to share your moms love and attention with anyone in the life partner role your Dad had before. And I have to believe that is a difficult thing. But let me ask you. Do you want your mom to be happy?
You say you are disabled and still live with her, and that certainly is a complicating factor. I suspect that might be a concern for you. Are you able to live on your own and simply choose to live with your mom, or does your disability require you to have a live in caretaker?
I can see that as being a friction point if not addressed up front.
On the issue about being a Step Father. My Dad was married to his last wife for over 20 years before I was comfortable referring to her as my Step Mom. Partially because they married after I was on my own, she never had a parenting role. And no she does not replace my Mom in any way, but she does add significantly to the blessing of my extended family. There is a difference between your parents spouse, and your step parent. Maybe not legally, but to me anyway, it was a comfort level thing...
Talk with your mom, honestly, openly and respectfully, and I highlight that last part in big bold letters, and express your concerns. And if you are struggling with jealousy, which is possible in this situation, please check your own heart. If you love your mom, you will want her to first things first, and that is follow Gods commands, and second and only after the first to be happy.