Thanks all for your replies. I do think my sister should dump him. I am not sure she has the strength mentally to do it now in the middle of treatment.
Re the Will - yes my sister has written a Will - last updated a year ago - leaving the house and any money in her bank accounts equally to her three adult children - now in their mid to late 30s. All three live abroad - two in the UK - one in Canada.
The bills in the house are all in my sister's name. These comments by her partner were initially made randomly several times over the years and then just recently. My sister when she was well told him in no uncertain terms that everything was being left to her children and that he had no right to anything from her as they were not living together.
My sister lives in Co. Galway - her partner stated that when certain people in the village died, people who knew them as neighbours/friends just came into their houses and changed the locks and then had to be paid money by the heirs to get out of the house. I think this is actually fact and this is where he gets his idea about changing the locks and claiming the house from. My sister was telling me a while back about a guy who used to work for a local framer, that when the farmer died, the labourer put a few cattle he had in on the deceased neighbour's land and claimed it as his own saying that he had used the land for years, but that was not the case. I think my sister's partner is going with this mentality.
I think the idea of the Garrison lock sounds like a good idea. Otherwise, in terms of the relationship, they have enjoyed doing things together, enjoyed the same things, country and western style dancing, gardening, meals out, until my sister became ill. At the moment she is holding her own, but I know her health could deteriorate quite rapidly. My sister asked me not to say anything to her children, but I feel she should be confiding in them about this.