This is your mil or sil presumably?
Unless there is some unwritten rule that family serve everyone else before themselves, or some cultural expectation that you are missing, it sounds as if they are going out of their way to make you feel uncomfortable op. Very unpleasant.
I think you always need to challenge this sort of thing in the here and now because they are relying on you saying nothing out of politeness and deliberately putting you at a disadvantage.
You can still remain civil but point out to them that you know that you are on to their tactics,
So when they questioned you about serving food you say very clearly and calmly, “oh I observed others helping themselves and you didn’t say anything to them so I assumed that was correct. Do you need some help? I am more than happy to assist?”
If they say yes then agree happily to help serve next time.
If they say no, then say something like “thanks for the reassurance and for making me and dd feel so welcome” and look them in the eye while saying it.
In other words, don’t allow people to be passive aggressive but ask them to pin down what is exactly wrong so they either have to own it or deny it.