Terminally ill sister wants to marry her money grabbing boyfriend?

My sister is in the late stages of a terminal illness to the point where she is in full time care. She is in her mid 30's and can't even speak anymore (she communicates through a computer). Her partner of approx 12 years on and off has always been controlling and a bit of a bully. They "reconciled" when her state deteriorated, but he gets a lot of free trips paid for by charities and has had access to her disability funds in the past. My family don't like him, although we are appreciative that he makes my sister happy and we don't want to cause any grief in what might be her short time left. Thing is, now he wants to marry her, but he wants to elope even though the care home where she lives offered to allow the wedding to take place there where it can be a day for my sister. We are utterly convinced its so he can be entitled to her money and anything else she has. At the minute it will all go to the Charity of the illness she has. Legally she is an adult. He avoids my family, although he was quick to help my grandmother with my grandad's life insurance details when he died and we are sure he has been in her house when she's been out. Also there's some of my sister's stuff disappeared over the years that was worth money, she doesn't even know whats happened to it. Itd 99% certain he's after her stuff, but is there any loophole in which we can maybe put a block or something so that he can't be entitled to inheritance when she dies? Bearing in mind he was out of her life for a long time and just showed up when she got quite bad and when they split up originally he took her for half. I feel sick to think he's doing what he's doing for his own gain. Any advice would be appreciated x