C.J.
Hi C. -
It always amazes me how many moms respond to less-than-glowing MIL reports. It's almost like an invitation to do some therapy. :) It seems that having strained relationships with MILs is more common than not.
While I wouldn't say that I'm exempt from this problem, being married for eight years to a man who has a difficult relationship with his mother and being the daughter-in-law of a very strong, independent, and opinionated woman, I've had to employ some very important self-preservation methods when dealing with her sometimes challenging behavior.
First and foremost, over the years, I've learned not to let my irritability get the best of me. When I actually have something ground-shaking to bring to my MIL's attention, I do so but only after I've prayed about my delivery first. When you're the daughter-in-law, delivery is EVERYTHING. Usually, when I deliver my requests in soft, even, non-antagonistic tones, my MIL accepts what I have to say (at least to my face). What's most important is that she eventually complies with my requests, which is my ultimate goal. Just as much as moms have issues with their MILs, the same is true in the vice versa - trust me, DILs are probably their FAVORITE topic of fodder and criticism to their friends and other family members. But who cares? As long as you get what you want - that is, for her to stop her critical and potentially destructive behavior, that's all that matters.
Second, ALWAYS take the high road when your MIL is being unreasonable. I have two young kids and am due with another in a couple more weeks. My first born, a four year boy, is the apple of his "Mammaw's eye" and always gets to visit her, while my two-year-old daughter is often left at home with me. While it does bother me to see them separated like that (my daughter DEFINITELY knows what's going on), I know it's better that she stay with me until my MIL can muster enough energy, patience, and understanding to watch both of them at the same time. That's not to say that she hasn't done so, but it's a real struggle and it would be for any person who doesn't have two young kids of their own. With your son being six and thus very cognizant of your MIL's blatant favoritism, it may be better that she doesn't take him at all. It could be potentially traumatic and miserable for him all around.
Finally, going back to my first point, just as we often justifiably brand our MILs as over-bearing critics of our mothering techniques, they are more-than-likely dubbing us as inexperienced and naive mothers. Far more than proving that you're the best mom ever, all that matters is that you and your husband know that you're doing the best that you can as a mother. As cliche as it sounds, YOU are the only one who can make you feel inadequate, not your MIL.
P.S. - By the way, your husband should get some nerve and talk to HIS own mother about how you're feeling, especially with regards to your son. My husband does even though it frustrates him to ever broach a difficult subject with his mother. Your husband is HER flesh and blood, and by virtue of that, she'll have more patience and a shorter run of her anger and/or irritation with him than she would have with you. He should position the discussion as "I've noticed that you've really been criticizing C. about the kids, and I'd like you to please stop. Also, it would be great if you would make an effort to accept C.'s son as your own grandson because he thinks of you as his grandma." I don't know any grandmother, blood-linked or otherwise, who wouldn't respond positively to a little guilt-induced accountability.
Blessings to you and yours.