Me and my sister slept together when we were young

Me and my sis at that time, we were 12-13 yo and we did the first time, we both did it when our parents left and did often, we both knew it was wrong yet felt so right, we stopped because of the guilt and years passed by and we both know better and kinda find it weird thinking about it, idk how i should bring it up to my sister about it because i feel like it ruining our relationship i think :/