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.
If the man who proposed to marry your sister is religious and has a good moral conduct then you should not refuse him on the pretext that the earnings of his father are forbidden and that she might eat from his father's earnings which are forbidden. There is no correlation between the two matters as Allaah Says (which means): {… And every soul earns not [blame] except against itself…}[Quran 6:164]. It is also confirmed that the Prophet said: "If a man whose religion and moral character are pleasing proposes to marry your daughter, then marry her off to him, otherwise there will be great mischief and corruption on earth." [At-Tirmithi] So we advise you to accept him if he is a religious man with a good moral conduct, after praying Istikhaarah.
As regards eating from the money of his father, if all his money come from forbidden earnings and he has no other income other than insurance, then the scholars stated that it is forbidden to eat from the food of someone whose earnings are all forbidden. It is forbidden to eat from it or take anything from it. Not eating from his forbidden earnings might deter him from earning forbidden income. For more benefit, please refer to Fataawa: 81560 and 85423 on the ruling of eating from someone's forbidden earnings or someone whose income is doubtful.
Allaah Knows best.