I am looking for travel Insurance advice for a family member who had cancer (ewings sarcoma)?

Me and my fiancee are looking to get married in Cuba next year, before we plan the wedding I want to make sure my loved one's can go too. My sister has finished all her treatment after being diagnosed with Ewing's Sarcoma, and now just has to get once a month check ups for the next five years. She is my maid of honor so I am not getting married without her and my mum there. I was looking for advice for insurance for her as it seems to be a nightmare, Hoping someone out there maybe knows more than what we do and get some good advice?! We are all Uk citizens, Thanks for reading my question.

please help because i really want to go