Is it a sin to refrain from marrying the daughters of one's paternal uncle who are afflicted with leprosy, knowing that five of them are still unmarried?

There is no sin in refraining from marrying one's paternal female cousins. However, if one has the ability to marry them and be patient with their shortcomings, this is an act of charity for which one will be rewarded, as relatives have a greater right to kindness.