Hi Hollycat, I was in a slightly similar situation to you, only child and in early fifties, when we decided to have my parents live with us (husband and son). At that stage it was only my dad in poor health and we wanted to look after them and support my mum. They have their own lounge and bedroom of course. In a rather large nutshell, my dad's physical health deteriorated and I took over looking after his medication. He had a major operation only a couple of months after we all moved, and really things went downhill from there. Mum was diagnosed with dementia two years ago, but looking back the signs were there and dad covered for her for at least two years before then. Mum ran around after him and so did I to help her. It got to the stage where I was worried about going out and leaving them and worried about what I was coming back to, as he sometimes had falls and mum didn't know what to do. I didn't know who to ask for help, was never asked about it by GPs who saw him, I think they just thought I was doing a good job managing. We did get to have a week's holiday after much soul searching, two years ago, I made lots of arrangements for relatives to come and check they were ok, lists of phone numbers etc, but as soon as we had gone, dad undid them all, followed mum downstairs one night and had another fall. When I cam back and saw his bruised face, my first thought was that I could never go away again for any length of time. Dad is now in a nursing home nearby and very frail. Mum's condition has escalated in the past couple of months and frankly I am close to the edge. We are organising help, but this is only going to be short term and it is likely that if possible, mum will join dad in the nearby care home.
We put everything in place that we can, but we haven't done anything spontaneous in the last six years. We have had two weekends away this year, but have no one to have mum for longer, our other relatives have their own health issues. I have not suggested respite for her because I believed it would make her deteriorate, plus the cost of doing that is just so high. My husband is a very good man, it was he who suggested this arrangement in the first place, and I believe he seriously regrets it although he wouldn't say so. Yesterday he kept account of what I do for my mum and even I was shocked. I'm having to work from home at the moment until we can put something in place so that I can be confident to leave her on her own. She has no short term memory at all and has to be told to do everything. My mum is an amiable lady, and that makes things 'easier' by comnparison with many others here on TP and I am very grateful for that. I wouldn't have done this had she been stong willed or domineering. She would hate to think of the effect this is having on me if she knew, it is constantly going round in my head at the moment and I find it hard to think of anything else if I am truthful. Don't underestimate the effect it may have on your relationship with your husband either.
I know everyone's situation is different, and therefore I wouldn't say to you on no account don't do it, but please, please don't feel obliged, because you are the only child. You are shell shocked, I know that feeling all too well, and that is no condition to make what will be a life changing decision. I echo what everyone here has said, and would suggest that you make other arrangements for your Mum to begin with and see how things progress.