I think if your OH OK'd their visit on the phone and is reluctant to ring and tell them no, then I would phone them myself and say 'I gather OH agreed to your coming here for Christmas this year? They will say yes, then you say 'Well, apart from the fact that it's far too early to be making plans for Christmas, in all honesty I found hosting you last time was too much for me, so I'm afraid I won't be able to do it again. If they come back with, 'Oh we'll help more, we didn't realise, etc', just say 'no, I'm sorry, but I just find having visitors for more than a few hours at a time too much for me these days, but if you'd like to get together for a drink or meal somewhere, then perhaps we can arrange something like that nearer the time'. Hopefully that will make things clear without being offensive in any way.