Is there a sin upon a couple if one of them has intercourse with the other late at night and they are sometimes unable to perform the Fajr prayer because of that? And is the fast of one who is overtaken by dawn while in a state of janabah valid in Ramadan after performing ghusl?

Leaving the Dawn (Fajr) prayer intentionally until sunrise is, by unanimous consensus, a grave sin, greater than adultery, theft, drinking alcohol, and taking a life. If one is overcome by sleep and misses the prayer time, there is no sin, but one must take the necessary measures to aid in waking up. If Fajr dawns while one is in a state of ritual impurity (janabah), their fast is valid if they intended to fast from the night, based on the saying of Aisha and Umm Salamah (may Allah be pleased with them both): "Indeed, the Messenger of Allah, peace and blessings be upon him, would awaken in the morning in a state of janabah due to sexual intercourse, not a wet dream, in Ramadan, and then fast." The hadith of Abu Hurairah, "Whoever wakes up in the morning in a state of janabah, there is no fast for him," is abrogated or interpreted as referring to what is more preferable.