Looking for Name of 80s Drama Film

I think it was a drama or maybe a thriller, can’t remember the whole plot, it would have been mid to late 80s when it came out I think, here’s what I do recall:

- The main characters in the beginning were two teenage sisters, one a couple years older than the other, they both still lived at home in a well-to-do family.
- The older sister had a boyfriend that would come around a lot and I think they were sexually active. I was an adolescent boy at the time so of course I remember the older sister had fairly large breasts, although I don’t think they were ever exposed in the film.
- I recall the sisters walking down a road together talking in one scene, either a country road or their neighborhood. No clue what they were talking about though.
- I don’t remember seeing this, but I want to say the older sister was hit by a car and killed. I know for sure she died, but I’m not sure how.
- In their grief the boyfriend and the younger sister start spending more time with each other and eventually get physical and have sex. The younger sister was a virgin, so when they have sex she is uncomfortable, even though she wants to do it, and then I want to say there’s some blood from the intercourse that she gets on her hand, and her dad walks in on them and catches them in the act.

Okay that’s about everything I remember, I just know this movie made an impression on me due to my age at the time and the risqué themes of the film. Now 30+ years later I still wonder what the heck it was I saw and what the full plot of the film is. I don’t know if it was just a relationship drama or if there was a thriller aspect to it where it turns out the sister was intentionally killed and then the killer comes after the rest of the family in their home later.

Any help y’all movie aficionados could provide is greatly appreciated! Cheers!