I am very much less favoured DiL. SiL has been referred to others by MiL as "the daughter I never had." MiL has no idea I know this, or how hurtful I found it.
Dh is second son - nowhere near as academic as first son, went down a more manual work route. BiL was more high-flying - uni, job in management, high wage etc. They are very different. It's not golden son territory, but older son fits in much more with MiL and married lovely SiL who matches well and comes from a similar background. Dh's job means he mixes far more with down to earth, work hard to make ends meet type people. It's like there's a class divide within the family!
My family is solid working class - not at all rough or loud-mouthed, but definitely a world apart. We scrimped and saved and held our heads up high to get through life as respectably as possible whilst getting by on the bare minimum. SiL's background is much more of a fit with how MiL is - both are, and always have been, solid middle class, working hard to be comfortable and for luxuries in life rather than to pay the electric meter.
I worked hard, first to go to uni, very good degree, professional job. I get on well with all Dh's family, but my family was and is very different both in terms of "class" and in the way we are towards each other. Therefore SiL and MiL fit together much more easily than I do with MiL. She's perfectly lovely to me, but there's a distance there - and we're probably both a bit to blame.
However, MiL is scrupulously fair to the grandchildren. Both families have two ch, both with at least one child with suspected ASD. All born within 5 years of each other. The children are all doted on equally. I suspect there may be a bit of preference for BiL's children - but equally, might just be paranoia. The children themselves have no idea. So you may find it doesn't impact any children you have.