Why can't you say how you feel? Saying what you really think IMO reflects the respect you have for them. If you're feeling that expressing your opinions is somehow harmful towards your relationship, then you should ask yourself, and them, do they respect you?
For me, it all comes down to how much I care about retaining the relationship. My relationship with family members goes beyond religion, and there are other cares, needs, and history that I feel I wish to maintain. It isn't all
about what I feel about God and their faith, there is more to it. And, yes, the ones I care about respect me, they just disagree with me, and they aren't a particular pain in the neck about it. Just the same, I find no reason to get into arguments with them, or discuss something that they aren't prepared to examine honestly -- and I am usually in no mood to have the same discussion again
for the 1000th time. Been there, done that.
In other cases, I can be much more dismissive of others if they don't like what I feel about religion. I have some family members of this kind who I don't care too much about. And I don't keep friends for too long if they are real
believers. I cannot respect their faith, and in most cases, they don't really understand why I don't believe. Those relationships usually disintegrate, anyway. But, anyway, I end up caring so little for those particular people's thoughts that I don't need to discuss it with them, either.
Generally, I am comfortable enough with my own atheism that I don't need anyone's input on its value. I also have no burning need to set any of them "straight" -- besides, any attempts of my own to do this in the past have ended with less than desirable effects, one way or the other.