Hello am I here,
My condolences on your parents' passing. Grief has nothing to do with how nice your parents were, but it has everything to do with how you felt about them. You built your bonds with them those times you visited them in the nursing home. You held their hands as they passed. These experiences don't happen without leaving a mark on you.
We grieve when we lose something that is dear to us. Your parents were dear to you, even if they weren't so to your siblings. It's natural for you to feel grief, it's just your body's way of processing what has happened. Suddenly there is no need for you to visit the nursing home. The people who were a part of your daily life are no longer a part of that life. You may feel a void since you have all this extra time without knowing what to do with it. It's a natural process. There's no need to question why you feel that way, but rather, let yourself feel it, let your feelings out and let yourself heal.