The halachos of kibud Av/Eim refer to bringing food to them, dressing them, and so forth. The halachos of MORAH Eim/Av (note the reverse order, because people naturally are in awe of their fathers, and so have to be reminded to give it equally to their mothers, but the reverse is true of kovod, where kids are generally more solicitous of their mothers and therefore are directed to show those same actions towards their fathers) are to not talk when parents are talking, to not sit in their seat, and not contradict them.
Derech Eretz should ALWAYS be shown to one’s parents (and others, but especially to the people who gave you life and raised you – or your spouse). I did not agree with my in-laws on many issues – they were not frum – but I always ALWAYS expressed myself and treated them with kovod and morah. They raised an exceptional son, and for that I have always been most grateful. When my MIL would offer unsolicited advice, i.e., how to take care of my newborns, I would listen carefully, assess her input, then do what I felt (or my pediatrician directed) was the best thing in a given situation. Sometimes her advice (like my mother’s) was excellent. Sometimes (like my mother’s) it was outdated, and I discarded it without negative comment.
My kids are the same today when I have advice for them. B”H most of the time, they have come to learn that my advice is on the mark, and they regret when they choose not to take it, which inclines them to follow it more frequently. I usually only offer advice when I am asked, but once in a while something happens and I blurt out a “you shoulda” because it is really obvious. When they listen to what I say, they see that I was right all along. Not everything a MIL or FIL or ANY parent says to you is wrong. Learn to separate the wheat from the chaff. BUT always be respectful. if you cannot be, then say excuse me and leave the room QUIETLY.