Not rude. If they all eat like that, it is their family way of eating.
It differs from yours. And is not how i would choose to eat, but they are relaxed with their family. And you chose to marry into this family.
Either dh and all his family have suddenly developed stomach wrenching table manners (odd to all develop together). Or you never ate with them before marriage (uncommon). Or your tolerance has changed.
If your tolerance has changed, why? Why does it bother you now, but didn't before?
Yanbu to find their habits unsavoury. But it is a relaxed family meal, not dinner with the queen (who I have read would tend to put fellow diners at their ease and not point out bad habits. But I'm highly unlikely to find this out for myself).
Your only options are to be terribly rude and make your guests or hosts self conscious by pointing it out. Or never eat with them again. Or learn to tune it out.
I suspect, like many noises, you noticed it once and now all the time it draws your attention. You can learn to ignore it if you try. Focus on what you love about the person. Focus on conversation. On the taste and mouth feel of the food. On the background music.
Or it will just irritate you more and more