I am on the fence about this, as a widow.
I think my husband would want me to be happy again, and that includes having another partner, although like most women I have not rushed into this, unlike so many men.
I don't think 5 and a half years is a short relationship, and I don't think it's disrespectful to your mum at all. I don't get this thinking, I'd be pleased he had someone as a companion and had some enjoyment and fun in his later years. Him being on his own all this time might have been very depressing and burdensome to you, and she's taken that on.
The other thing is, she's not the poorer one here by all accounts. Her children are probably worried she's marrying him. If they both have houses, and they don't currently live together, it sounds to me like they are planning to downsize or seek care together so they are not living separately for this later part of their lives. I think that's a noble aim, and speaks well to your dad.
You can discuss his plans with him and talk about inheritance and POA and how you don't want to be locked out of health decisions, but my guess is that he does want that commitment to her and that will include some aspects of her deciding on his health and him on hers, and some sharing of finances.
I would tread very carefully here OP, you are sounding quite mercenary here and he may indeed not care about that at this point in his life when he's planning how to care for her, himself and how to make sure their money makes their lives more comfortable.
My priority is to live a good old age, have the appropriate care and have someone I love by my side, he may be the same. If that means my children inherit less, so be it, although I would do inheritance planning so they receive something.