I had a short relationship with my sister and brother when I was about 13 to 15. My brother and I would jerk each other off (he is one year younger than me). We also tried oral sex but it was not that great. My sister is two years younger and we played with each other’s parts at various times until we had fumbling missionary sex. My mother found out and went gonzo which ended it.
I suffered no ill effects but my sister may have been overwhelmed by parental reaction.
I do remember it was great to cum in her and would love to have another opportunity. We are on opposite ends of the country and I doubt it will ever happen again but I still fantasize.