How do I change custody arrangement for my daughters to be sole or joint?

I have two girls age 9 and 11. My ex has sole physical and we have joint legal custody. I am in Mass and over the past year+, my ex has been bed ridden with an illness that can not be identified. She classifies as terrible head pain. Of which "NO" medical proffessional has deemed a reason. Yet she has been medicated and in bed around 90% of the time for over 18 months. This has taken an adverse effect on my daughters and they both request to live with me and my new wife and younger daughter. I have always maintained a good relationship with my ex and her new husband, with no issues of visitation. It is now that I know the well being of my daughters must be addressed and the current arrangement no longer benefits them positively. I have numerous questions. Any help I would be greatful.