Yes, it most definitely is!
> Just because someone might conclude that advice is wrong, does that mean family members are out of bounds for giving that advice?
No. But "just" because your "advice" is completely unsubstantiated, which you would notice if you cared to exercise a minimum of skepticism, that does mean that giving that advice is out of bounds.
The equivalent of pushing religion is not "breast feeding is healthy", the equivalent is "you should not vaccinate because that causes autism". Outright bullshit is not the same thing as possibly not 100% perfectly reliable science.
> Or that people shouldn’t have to graciously accept it?
We are obviously talking about a situation where someone in the family has made a conscious decision to leave the religion behind. That is obviously not a situation where they should be expected to graciously accept any further pestering on religion, because that is obviously overstepping a known boundary of that person.
(Note that that is different from trying to talk to them about religion, where there is genuine interest in understanding their position, rather than just the intent to convince them to come back--that is perfectly fine as long as the person does not explicitly state otherwise.)