Does thinking about having sex with her brother make a girl not his sister, and her family not her family?

Adultery is a major sin, and its heinousness increases if it occurs between unmarriageable kin (mahram). Allah has threatened severe punishment for those who commit it. Allah Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." And Allah Almighty says: "And whoever does that will meet a penalty. The punishment will be doubled for him on the Day of Resurrection, and he will abide therein humiliated."

The punishment for incest is death, based on the saying of the Prophet (PBUH): "Whoever has intercourse with a mahram, kill him."

Nevertheless, this abominable act does not negate a person's lineage from their family, nor is their lineage negated merely by thinking about it.