I'm trying to remember a YA novel I read back in the 80s/90s (in English) about a teen girl who goes to spend a summer with her grandmother or other older relative, something weird timeslippy happens, and she's able to slip between present day and the 60's I'm guessing [?], and becomes friends with a murdered relative [I think grandmother, but maybe older relative]. Along the way she changes some stuff [an elderly neighbor is blind in present day, but when she goes back she stops the fireworks accident from happening to him on the beach when he's a child], in fact at the end she discovers she now has an extra family member and some houses are painted different colors now. Anyhow, she also ends up stopping the murder. Ring any bells?
You're looking for Whispers from the Grave by Leslie Rule from 1995 about a girl who discovers an old diary of another girl that suspiciously resembled her and was murdered, and she goes back in time to prevent it. Everything you mentioned is in there (including her preventing her neighbor from using the firecrackers that blinded him), though the murdered girl was not really an old relative but rather
her twin somehow!
You can find the paperback here.