Honestly, I WOULD get involved for a couple of reasons, the primary of which is he's your father as well, and you have equal say in his safety, health and comfort. Secondly, your sister brought you into this situation by way of telling you her thoughts, which then allows you to state your thoughts in rebuttal. I were in your shoes, I'd most definitely 110% make sure my opinion was known, unpopular as she will think it is. Frankly, I'd also not be particularly polite about it at all and not hold back. Sounds like your sister needs a verbal slap in the face of reality.
Your father's health and well being are much, MUCH more important than a memorable photo op. Many apologies to those posters that treasure those memories (I agree that those pictures must be lovely, very much so), but I would put my Dad's health before my desire to have him attend a wedding. As much as I'd love to have him there, I would understand why he wasn't able to participate and I certainly wouldn't begrudge his decision.
The very idea that your sister wants your father to potentially risk his health and well-being simply because he can be at her son's wedding is incredibly self centered. Somehow, given what you've said about your nephew's relationship with his grandfather, the groom to be likely wouldn't care. It's your sister that does. It's not her flippin' wedding.