Explaining firsts is tough. When you've had invest experiences especially when they were your first time doing x and a partner asks you almost feel like you have to lie. Then you can't change the lie later in life. Imagine you're at a family picnic and your partner is sitting there talking with your sibling(s)/parent(s) like all is well and normal. Completely unaware of the sexual nature of your past with each other. The frequency of those events due to living together, the learning new things with each other, exploring kinks and the likes. Imagine them knowing the thing they really like in the bedroom was something you first did with a sibling. Fortunately some people find partners who are open to incest and have their own experiences. others may not find this in their lifetime. While many of us have fantastic and positive experiences with incest, it doesn't always extend to our partners.