Who can claim adult child on tax return

My 25 year old daughter has filed for disability and it's still in review process. 

She lived with me for about 3 months this year and she lived with her aunt the other months. My ex-husband has been paying for her health insurance, co-pays, medication. However, he wants me to pay for her health care expenses for October, November, and December. He said he will claim her on his tax returns. I need some advice please. Who can claim her on their tax returns? Him? Me? Or even his sister?

Thank you.