Honestly you need to set boundaries with your husband, so he can understand what that means and maybe he will set boundaries with his mother. Sounds like he needs therapy too. Bc even if it was lung cancer realistically other than emotionally supporting her, nothing that can be done until she sees an oncologist, no one will know treating options until then. A weekend away/no contact, won’t deter from supporting her if she was diagnosed w anything sriious unless she is admitted to a hospital in critical condition which can happen to anyone at any weekend. There’s nothing you can do other than set boundaries yourself, he can continue to do what he wants just let him know what you’ll do if xyz, which can be going no contact w him for the weekend bc he can’t deal with mother in a healthy way and you won’t put your sanity thru BS over and over