Watching those kind of materials are zina of the eyes
Narrated by Muslim, 2657:
Verily Allah has fixed the very portion of adultery which a man will indulge in, and which he of necessity must commit. The adultery of the eye is the lustful look, and the adultery of the tongue is the licentious speech, the heart desires and yearns, which the parts may or may ...
You can not marry. His mother breastfed you so he is now your brother. It does not matter if you shared with his sister or someone else. There is no solution that can make it lawful for you to marry each other, just find someone else.
Just go for it your sali is good after death of ur wife no matter how young she is. You are lucky person to have such great opportunity with the permission of parents just marry her brother and make her happy on first night.