Barbara Kingsolver's book High Tide in Tucson has a wonderful essay on divorce. Her comment was that such marriages shouldn't be called failed marriages, but finished marriages. And offspring (her own child for instance) sometimes count as a benefit the greater diversity and larger family that might result. Her daughter loved having three grandmas.
The marriage nurtured you and your sister to adulthood. There shouldn't be any guilt if they choose to go on to new goals. Do you expect your stepfather to not be there for you and any children you may have? Might he be afraid that you will be the one to cut the tie?
These are just random musings because I've no experience with divorce in our family (amazing in this day and age, I know), but Kingsolver's essay was not only insightful but most enlightening. Gave me a lot of food for thought about others who do find themselves in this situation.