I hate to say it, but I probably would've sent the family member up the river, for a few reasons:
1) I've dealt with a lot of alcoholic/addict family members and have gotten a better understanding of tough love in the past few years.
2) I feel that if one of my family members needed a kidney I would absolutely be at the hospital getting tested, but a relationship takes a lot more than DNA; it takes mutual respect and care, something Sara's father obviously doesn't have.
3) For a father (word used loosely) to act with such little concern for their child's future or well-being, he no longer has the honor of being a parent, and has reduced himself to being a glorified sperm donor.
4) I might be a cold-hearted "B."
The truth is you have to do what lets you sleep at night. I can guarantee though, when Sara finds someone who wants to be a partner in her life, they're not going to allow that to continue. Who in their right mind would let someone, who isn't even an active part of their significant other's life, infringe on their ability to buy a house or start a business?

I sincerely hope Sara begins to value herself enough to take care of this matter, even if it involves the police. He had to have forged her signature, etc., and how is she going to feel when he does this to her little brothers, and gets off easy because he has no "prior acts"?