A young man proposed to marry me and he is younger than me, I know that this is not forbidden, but my question is: Is it permissible for me to marry the son of the sister of my ex-husband who divorced me?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

There is no harm in marrying the son of the sister of a man who divorced you provided the conditions of a correct marriage are met and provided there are no marriage impediments. The son of the sister of the husband is not a Mahram (a non-marriageable relative) unless he is a Mahram to you by suckling relationship, i.e. if he suckled five definite times or more from the same woman you suckled.

To conclude, there is no harm for a woman to marry one of the relatives of her ex-husband after her waiting period finishes [after he divorced her]. 

Allaah Knows best.