transferred to her when she reached 18 or 21, depending on your state.

You could send her a text about the health insurance issue. Hopefully she will respond. Then pay the premiums directly from the custodial account. Like you did with the car repair.

A reasonable person would agree that you are using the money in her best interest. Giving her the money without being able to control how it's spent is obviously a bad alternative. If in the future she gets her life under control, she would be grateful.

Perhaps you should get some legal advice on this.

Depending on states, age 18, 21, or 25 would be when the custodian is required to notify the lucky recipient. Was the custodian issuing 1099s? Quite an oversight if they weren't. She could be subject to back taxes, the custodian could be sued.

Agree with the other posts, consult a lawyer, one who has a CPA on call. I'd also ask deeply about a financial planner to see what options are available for you (if any) and for her.

In my view a big carrot like that and some arm twisting might bend the situation a bit to fit your goals.