Luckily for my DCs, we lived and still live a long way from the GP who thought the sun shone forth from certain grandchildren's rear ends -- two children in a family of three, with the third child relegated to 'odd child' status and all the rest of the fifteen grandchildren were 'Also-rans'. This favouritism was not expressed in material terms but in terms of glowing praise of every single school report, every school play, every piano recital, every sports day, etc. Nobody else got a look in. Great things were openly predicted for these children, especially the oldest.
If it's any consolation to you, OP ( schadenfreude alert ), the older Favoured One flunked out of university and works in a shop now. The other was proclaimed to be the next Einstein but it turned out he was in exactly the same level of maths classes that all of my children took all the way through school and is interested in the same sort of subject in university that DD3 is interested in. They are the same age but GP (ok, it's exMIL) has never once asked what DD3's interests are, what subjects she is good at, etc. The 'Odd Child' quietly excelled in school and chose to apply to a hippie university on America's west coast, graduated, and now has an excellent job. The rest of the grandchildren, my children included, did very well in school or are progressing well, got into good universities, and graduated or are heading towards that goal with no dropping out in sight.
The Favoured Ones are nice people. It isn't their fault exMIL openly worships the ground they stand on. The way the family operates, enjoying exMIL's favour was seen as a good thing by the mother of the older Favourite and so this child was never protected from exMIL's tendency to praise results and not effort, and learned to suck up and work only if lavish praise was going to be the result. Middle child ('Odd Child') had a mind of her own from the start, and the dad (Son in law of exMIL's) had learned from oldest child's experience so he was shielded somewhat from exMIL. I do not know how it has all affected the relationship between Oldest, Middle and Youngest children in that favoured family. The mother (exMIL's DD) is very dependent on her mother (exMIL) for validation, approval -- emotional crumbs from the table really, as her next younger sister was always the favourite child when they were growing up. What a poison favouritism is.
My DCs live effectively without a grandparent as my mother is in another country and only visits every few years, and we were lucky not to have a lot of contact with exMIL. They get along very well with my mother and enjoy her visits.
Whatever way you can find to shield your children from all of this, please do it, and do not share with them anything you hear or learn of, or your own anger or disappointment. Behave as if everybody in the universe loves and cherishes and appreciates your children. You can reveal the truth to them when they are much older or when they notice things themselves and ask, and even then, be brief and as neutral as possible in your answers. You can have a right old bitching session when they are in their late teens, early twenties..