"so that no one can find fault with them." Possible meanings of "they" are 1) "these widows and their families" or 2) "the believers." It might be best to leave the subject as "they."
"is worse than those who do not believe in Jesus." Paul means this person is worse than an unbeliever because even unbelievers take care of their relatives. Therefore, a believer should certainly take care of his relatives.