Okay.. very good points raised.
But now people can use condoms.
So tell me why can't mom/son , father/daughter , brother/sister can't have sex with each other just based on morality.
Morality is relative... so ?
So, here are my thoughts on that. There are broadly speaking two ways for people to be related to each other: social contract and genetics. Social contract families are the ones you spend your formative years with, I'd say roughly until your mid-20s (but at minimum until the local age of consent), the ones who raise you. Genetics families are the ones who are related to you by genetics, your biological family.
We live in a very interesting age where it is possible to be related to someone via genetics and never know. We have sperm donors who are genetic parents to hundreds of children, and very often those kids have no idea they're related. On the flip side, we also have social contract families who don't necessarily know their genetic background (are they adopted, children of sperm donors, etc).
I think the important family members when we're talking about incest are your social contract family. The reason why I believe that is because they're the ones who it will always be a power play to have a relationship with. From the day you're born, your brain makes you predisposed to do and think what your parents tell you and other authority figures in your life tell you, a consequence of our evolution (the kids who do what their parents tell them and don't try to pet the lion are the ones who survived). Within a social contract family, there is always going to be a hierarchy of power, and so incest within a social contract family will always be an abuse of power on the part of one of the participants and therefore the ability to give consent is compromised.
However, if you have two genetically related people who don't have a social contract family--like, say you've got two people who are children of the same sperm donor--and they meet as adults at a bar or on Tindr or whatever--I don't see a problem with them having a sexual relationship, as long as they don't reproduce. They'd probably have trouble with fertility and might not know why until they went to get testing done, maybe even after being married for quite a while, and in a case like that, I'd say it's fine, no need to make them split up, just tell them to either adopt or use a sperm donor to have kids.