I think my step-sister was my first kiss
What makes you think your step-sister was your first kiss?
I really don't know it's just a strong feeling of deja vu.
This may be difficult to do but, you should steer clear of having intimate interactions with your step-sibling. This will potentially affect your family in a negative way. It can also destroy the relationship between you and your step-sister.
Sharon doesn't talk about it anymore and I don't think she cares.
You should watch out that it does not occur again and continue to let it not come up.