I'm still suspicious of the dentist! Probably without good reason, but the whole storyline with him so far just seems a bit odd. Either badly done or there is more to come...
Firstly, he seems kind of smarmy, just the type of man to be an utter wanker. Maybe end up raping/being otherwise violent towards Trixie. That may just be the way he comes across though. When he appeared to be cheating I thought it all fitted in nicely. Then Barbara said something about men only calling a woman "darling" if they really mean it - which set of a clanging naivety alarm for me, I assumed he would turn out to be dodgy!
Also, re. sister Monica Joan's dementia... I'm just thinking, if she was the thoughtful poetic type she may have fallen into rambles that were incoherant/odd to others long before her mind was seriously affected...? I know when Im thoughtful I could easily ramble on and sound quite mad; it would only take a small reduction in social awareness to appear quite different.