My XMIL had 3 sons and always blatantly favored her youngest son and his 2 kids over her other sons and her 7 other grandchildren. All grand kids the same sort of age.
All 4 families lived within walking distance of each other, but the only house she would ever visit would be the youngest son's, and the rest of us always had to go to her. And i mean always. If we didn't do our duty and turn up often enough we were 'talked about' (it would always get back to us) and get the cats bum face for a few weeks. She and FIL shelled out pots of cash for only those 2 GCs to go on expensive holidays with them, and much more time and money was spent on that son and his family generally. It was so obvious it was almost funny. Almost.
My DCs are my parents only grand kids, and although they never had the money to throw around like PILs, they lavished time and love on my DCs, which my kids cherished, so i don't think they ever really felt the unfair situation going on with 2 of their cousins and the other set of GPs.
I think it helped that XH and I always played down the situation. It was obvious how things were going to play out from early on, so we just made sure to try to avoid putting our kids in situations where they would have it rubbed in their faces. Difficult though, when MIL seemed to delight in making a point of making a big thing about where she had taken their cousins recently or about the new stuff she had bought them (making my DCs sit down and plough through a photo album of the recent trip their cousins had taken to Disney World Florida, with grandma, being a particularly low point, i remember
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With hindsight i can see that MIL was particularly attached to her youngest sons kids, because right from the start she had lots of control over their upbringing. SIL was happy to use MIL as a surrogate mum and their dad (the golden son) was a crap husband and father frankly. Perhaps in her own misguided way she was trying to make up for this by spoiling those kids so much. Sadly it meant she has basically lost contact with her 7 other GCs though (they are all teens now, so able to make their own decisions) as their was very little love or bond between them all.
You reap what you sow, OP. Your MIL has more to loose than you or your DCs in the long run.
(loooooong post, sorry
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