Hi Elle,
I really feel for you. As Bunpots mentioned, we had a very similar scenario, with my Mil utterly convinced that her son was her husband. This started several months after she came to live with us, and it was horrendous to cope with. When it first started, we would laughingly correct her and she would accept it - and be mortified, poor thing. But it snowballed pretty quickly, becomming more frequent and her becoming increasingly adamant about it. Till we got to the stage where absolutely nothing we could say would sway her from this belief, when this particular delusion was in full swing (which was often). We had the innapropriate behaviour - inviting him to 'come to bed' (in front of our 14 year old daughter!) , trying to give hugs and kisses that were definitely not the mother-son type displays of affection. What was just as concerning, however, was how it shaped her attitude and behaviour towards me - because if he was her husband, then of course, I must be his 'bit on the side', the 'other woman'. She would explode (sometimes out of the blue) if he he called her 'Mum' or me 'love'. She would get furiously angry at what she perceived to be signs of his infidelity, not liking it if we sat next to each other, spoke to each other, if he kissed me goodbye or hello, sometimes even if we only looked at each other. Can't tell you how many times, even when he wasn't around, that she would start asking me how could I 'live with myself', knowing I was 'carrying on' with HER husband. And that would quickly lead to long and sustained bouts of verbal aggression. As I was her main carer, you can imagine how impossible that made life at times.
We could show her our wedding pics (she would claim that they were from 'just a party' or even that he was married to me 'years ago' but we had divorced and he had then married her), our marriage certificate (that was a fake, apparently). We went through a very strange stage where she would acknowledge he was her son - but claim he was also her husband, that they had got married in Ireland, where it was 'allowed'! She would stand there, screaming at me that he couldn't be my husband - because he spent every night in HER bed. 'Cringe' was too mild a word, at this stage! We would try to reason, to explain - all pointless, we discovered. And for obvious reasons, this is an instance where compassionate communication is absolutely no use at all - innapropriate talk, actions and verbal aggression are not something you can go along with in a situation like this.
In the end the only way we could deal with it was to cut her off as soon as she started. Correct her, refuse to discus or argue, and tell her to go to her room - there were times when OH actually escorted her to her room. Just her being near us was enough to feed and sustain the delusion for however long each incident lasted for. Taking her away from us, out of the environment was the only way that maybe, just sometimes, the delusion would stop - till the next time.
There were other issues, caused by extreme paranoia, delusions and sundowning that eventually led to her being admitted and sectioned. The hospital changed meds, withdrew meds, added meds - but still, when we visited, this delusion would occasionally appear, along with all the others. When we tried to bring her back home, within 5 hours, it happened again - and this time she physically attacked me. That was it. She went back to hospital, and from there to an EMI Nursing home. And even then, there have been times we have visited when this delusion has re-surfaced, and all we can do is leave - even if we have only been there 5 minutes - when she starts. In my experience - though I really hope its different for you - no medication tried had any impact. This delusion, the delusion where she believed there was a missing child or baby and she would be frantically trying to find it, the delusion where she believed there were terrorists and gun men all around her - mediction didn't help with any of them, they have only lessened with time and her deterioration.
I know how horrible it is, how guilty you feel when you have to restrict the length of visits because of something like this. At one stage, I even stopped visiting for quite a few weeks, and for several months after that, I would only visit with OH, because if he wasn't there she often was even more verbally aggressive and unpleasant with me. I just had to keep reminding myself that her getting upset and angry, often working herself up into a state where her anger made it more difficult for the carers after I/we left - well that outweighed the guilt. It was better for her if we restricted visits because it lessened the upset and agitation. Easier said than done, I know. But although perhaps your Dad's GP may find some medication that will help, or the mental health team (if you can get their input) might be able to come up with alternative stratagies (Mil's CPN couldn't!) , if it turns out that nothing but time will stop this delusion, I think you have to prepare yourself to cut down on visits, and keep them short till this delusion eventually, hopefully fades. These days, Mil usually thinks that OH is her brother - sad, but not asd hard to deal with as the husband delusion.
Good luck, and I'll be thinking of you. It really is a dreadful and difficult situation to try and cope with xxxx