Suspected forgery: do I do something or leave it alone?

This is a real-life ethical dilemma. Please read the facts below before offering advice.

1. My relative Roy has two daughters, Ellie and Sarah, a year apart in age. He gained custody of them in his divorce 7 years ago. In my opinion his wife did the right thing to get out before he hurt her physically.
2. A year ago Ellie graduated from college. I sent her a card and a check for $100. I never received a thank-you, but wrote it off to youthful thoughtlessness.
3. Sarah has just graduated from college. I received an announcement hand-addressed by Roy, with his return address—and a small envelope enclosed (just big enough for a check folded in half), addressed to her at his home.
4. Neither daughter is living at home at present.
5. I don’t have Sarah’s current address with her boyfriend in another city. Her official address is her mother’s home in another state, but she isn’t there.
6. A week ago I learned that Roy is seriously in the depths of alcoholism, desperate and suicidal. I had no idea. He is apparently an angry and abusive drunk and his daughters are afraid of him. He’s unable to hold a job. I don’t know what he’s living on.
7. Before writing a check (to send with a graduation card—not in Roy’s tiny envelope) to Sarah, I looked up my check of a year ago to Ellie just to make sure it was cashed. Seeing it for the first time, I was shocked. The endorsement does not look like the signature of a 20-year-old girl. In fact it looks like Roy’s scrawly handwriting, similar to the way he wrote her name with his and Sarah’s on a Christmas card two years ago.

It would be easy to trap Roy by sending a small check made out to Sarah in the prepared envelope and sending another directly to Sarah in care of her mother, and comparing the signatures. Forgery and theft of checks are a serious business.

Should I? Should I go further and make it a legal matter?

It could kill him. It could save his life. It could tear the family apart. The family is already torn apart. In my mind the only reason for doing it is that it could force him into sobriety and a new way of life when he has reportedly refused all help. I know that in some locations alcoholics may be given a choice of going into rehab instead of serving time, or instead of serving a full sentence.

But I see that taking such an action could do more harm than good. And this is at the other end of the country and could mean a lot of expense for me if I had to travel and stay there (would I?), a burden I’m not really prepared to handle. I also see that he is apparently forging his daughters’ names on checks and stealing their money, and that should not be condoned by silence. However, even an open accusation without legal action could precipitate an unmanageable crisis, especially for the daughters, who are innocent victims.

Do I have an obligation, and if so, to whom?

Should I just let it lie?