Whatt o do about an injury claim against a homeowner's policy?

First, you do not sue an insurer--you sue the homeowner. While you can ask the homeowner to submit a claim to her insurer, she is not obligated to do so; and even if she does, the insurer will only pay if it believes it a valid claim which they'd lose on if sued. So if your ex-husband does not receive reimbursement or compensation voluntarily from your sister or her insurer, he will have to sue her for the money--at which point her insufer should step in to defend her.

Second, from what you write, it is very unlikely that there would be any liability. A homeowner does not become liable for injuries simply because someone was injured on their property; they are only liable if there was fault. Your daughter was, presumably, voluntarily wrestling with her cousin, which means she was voluntarily engaging in an activity with a reasonable probability of injury. Unless you can show that the cousin was either acting with reckless disregard for possible consequences--for example, leaping on her from high furniture; not letting up after she yelled that he was hurting her; etc.--or that he deliberately injured her, there would be no liability. Sometimes people get hurt while wrestling without it being anyone's fault.

If you can get past the two hurdles above, then in theory he could sue to recover that portion of the medical costs for which he is legally responsible.