What does Islam say about Adam and Eve's children and how did third generation come into existence when Adam and Eve only had 2 sons?

Concept that Adam and Eve had only two children is wrong. They had two children and two daughters. One daughter and One Son being born at the same time as twins. So there were two pairs of twins. Members of one twin pair married the ones from the second twin pair and hence the generation continued. Quran clearly metions the birth of twins at many places. Although, coming together of siblings may seem like incest, it was allowed at that time and only later was it made a sin. The reason God later outlawed it was because the genetic degeneration of the human race (Adam had none originally - it began when Adam and Eve ate the fruit of the forbidden tree - tree of the knowledge of good and evil), had reached a point where there was the increased possibility of the children of such union having disastrous genetic problems (we know this to be true medically now, but when God forbid such union, He did not explain why, but simply forbid it - the reasons are obvious).

This is also verified by Old Testament that says in Genesis 5:4 that the children of Adam & Eve married their siblings, although it does not accept that the siblings were twins.

However, some people dispute this Quran and Testament backed theory and instead maintain that since incest is considered a grave sin in Islam, this is probably not true. They believe that celestial bodies descended upon Earth to be brides for the two brothers or that the female twins gave virgin births similar to that of Mary.

i dont understand this virgin births thing- within Christianity Mary is singled out for giving birth as a virgin

she was the only one in history to do so- by saying that the twins gave virgin births is contradicting that