What would you do if...?

first of all you wouldn't be marrying your blood sibling. You would be marrying someone who so happens to be an off spring of your step mother.father. No relationship here what so ever. This may put all concern in a uncomfortable position as family identification is concerned a minor problem. But that's about all. It wouldn't be right of of the off spring to interfere with their respective parent engagement to each other. Nor should they interfere with your engagement. Respect and love is the key here.