To isolate mom and her bias, speak steadfast truth to family
Q: Both my brother and I are close to our parents. (We also like each other.)
However, my mom plays favorites with each of our households. She constantly complains about the way my sister-in-law (who is probably my best friend) keeps house, cooks, shops, dresses, etc., while turning a blind eye to the realities of my life. Even when I point out, for example, that my house is usually messy by her standards, she comes up with excuses for me. I find myself looking for ways to point out what I'm doing wrong just to even things out. She denies the favoritism.
The worst part is with our kids, though. She favors mine, and does it obviously. Recently, when my sister-in-law told my mom about something wonderful my niece had done, my mom replied with something like, "That's great, but did you know X did such-and-such?" There was no need to bring up my kids at all at that moment. I know it hurts my sister-in-law and frustrates my brother. What I don't know is how to fix things.
My sister-in-law and I do talk about it, and we both shake our heads at the unfairness. Soon, though, the kids are bound to notice.
A: "Fix things," that's a problematic choice of words for someone in your position, at the periphery of this problem.
If you set "get Mom to behave" as your goal, then you're not only bound for disappointment, but also you also risk diverting your energy away from a more realistic goal.
And that realistic goal is to give Mom zero validation for her favoritism. You will have none of her bias-affirming games. When she complains about the way your sister-in-law keeps house, for example, don't just say, "Gee, Ma, my house is messy, too." That has already failed to dent your mom's prejudice, for one thing. More important, it's also tacit affirmation of her dim view of your sister-in-law's worth.
Instead, mount a defense with truth and teeth: "Mom, you're talking about my best friend, and I won't be a party to criticizing her"; or, "Mom, she's a lovely person and we're lucky to have her in our family"; or just, "Mom, that's unfair."
When you witness her dismissing your niece's accomplishment, you voice the praise your niece deserves, and then remind your mother (either lightheartedly or privately) of what you correctly observed here, that there was no need to bring up your kids at all. Remember, your mom sees the sun as shining more brightly on you; don't underestimate the power of your disappointment in her holds.
And, don't keep shielding her from that disappointment. Comment by comment, kid by kid, visit by visit, strand your mother on her little island of bias.
For her sake, I hope she recognizes these consequences and adjusts her actions — but for the sake of your brother and his family, they just need to see your mom on that island, with no one condoning or agreeing with her slant. Not the ideal outcome, but all they really need.
That's because isolation diminishes her power. When she's nothing more than an unfortunate exception to the family's warm and mutually respectful rule, her grandchildren won't think, "What's wrong with me?" Instead they'll be wondering, "What's wrong with her?"