Wife who I left 4 years ago due to her cheating on me. We never divorced. She went to the hospital in a diabetic coma a few days ago, and her sister texted me yesterday saying that I need to pay half the hospital costs or she would have the police come after me. Am I legally required to pay it?
EDIT: Thanks everyone! I will speak with a lawyer for advice, completely ignore my SIL, and get divorced ASAP.

Probably not, but a lawyer should advise you. It may be similar to how parents owe child support for kids attending post secondary school, except if the child made expressed actions prior to end the relationship with the parent.
Also as a husband I would pickup my wife’s expenses, however there’s no law saying you have to. Each spouses debt is their own.