Okay, so I'm thinking about the Polnareff siblings in relation to the fact that their mother died when J.P was young.
Things to establish(using the 1987-8 timeline):
J.P is about 23-4 in SDC, and Sherry was a high school student when she was murdered in 1985.
If we subtract the years here, Polnareff was about 21 when Sherry was about 15-18. This gives us three possibilities for the sibling age gap. Pol could have been between 3-6 years older than Sherry.
This is all important because it gives us a timeframe in which their mother couldn't have died. This would inform how much of a brother/parent role J.P would take for Sherry.
Further into speculation:
I'm leaning towards a 5 to 6 year gap, this is because it makes it that much easier for him to lift up Sherry and spin her around in the one flashback we see. Unfortunately, this means that Sherry would have been 15-16 when she was murdered (which breaks my heart).
For the sake of my heart, I'm going to say the Polnareff siblings lost their mom when J.P was 8 and Sherry was 2-3. I think their mother's death was a factor in how protective J.P was of Sherry, and how close they were as brother and sister.
Suddenly, there were only three people in the house. Suddenly, J.P has to become more responsible for his sister, make sure she's taken care of and happy while their father is working. I just have the feeling that Sherry might have thought of J.P as more emotionally available and dependable than their father (regardless of how good or bad a job their father was doing, since Sherry and J.P had a similar experience ((losing their mother)) rather than losing a wife, I think that's something that would cement a different type of understanding between the two of them).
Obviously every sibling bond is unique, but I think J.P and Sherry's bond makes her death just that much more...compoundedly awful. Because it isn't just his little sister dying, Sherry was probably his best friend, the person he was closest to, the only other person who understood the pain of losing their mother (who would probably ask: what was she like? do I look like her?). Someone he had to grow up that much faster for.
It just makes my heart ache.













