Is a man who has sexual relations with his sister a dayyuth who will not enter Paradise?

If by "intercourse" what is meant is fornication with one's sister, then this combines both fornication and "dayouthah" (lack of protective jealousy over one's family and mahrams). The person who commits this abomination must be advised to make sincere repentance. If they do not repent, then their family or someone who can restrain them must be informed. The warning that a "dayouth" will not enter Paradise can have two meanings: either they will not enter it at all if they deem the act permissible, or they will not enter it initially. This warning applies specifically to those who die while persisting in this sin without repentance. As for the one who repents, Allah accepts their repentance.