I have a SIL and BIL like this, no matter what we say we're doing, not doing, they poo poo our parenting choices and make us feel as though we're always 'wrong' because it's usually always in opposition to theirs. Even when we baptised our children they went behind our backs and mocked us.
Like you Little Boys Blue, we get on with them really well the rest of the time, it's only when 'parenting choice' issues come up that we find they don't show us the same courtesy as we do them.
I don't initiate any conversations with potential 'provocative' parenting choice topics, however whenever we are with them 9/10 conversations turn that way because they will initiate it and its' almost liek they get a kick out of doing so.
I have no advice, but I do empathise , I guess oneday it will all come to a head (which I don't want), so I guess either DH or I have to bite the bullet next time and gently say something.