Should we interbreed with our family members?

Soluzar is exactly right. Mutations occur randomly regardless of whether humans can control their environment. The earth itself is pretty radioactive and then there's cosmic radiation. Both of these can cause spontaneous mutations with no actual NEED for adaptation. So, you might get a situation where in some people, the appendix is missing. Since humans never needed that anyway, the loss of that organ makes no real difference to their lives except maybe for a slight risk to health if it ruptures. But because humans can easily cure complications caused by a ruptured appendix, evolution won't filter out those who have this weakness (i.e letting them die while the ones lacking an appendix live).

Humans can also compensate for people with physical and mental disabilities too, so these people will not die out despite being technically weaker and less able to survive in the wild. I think the reason why humans are growing taller (notably Asians in particular) is because of different, more Western-style diets that give them better basic nutrition when they're younger. I don't see any environmental need for them to be taller, so I don't think it would just happen except by accident.

Jam it back in, in the dark.