This is so hurtful, OP. Parents have the responsibility to ensure that their children are treated fairly and equally, as much as possible. And if they have favorites, to keep it to themselves. There is very little that hurts and destroys trust more than witnessing blatant favourtism and subsequent gas-lighting. To have it happen to your own children just demonstrates how entrenched and selfish their behaviour is.
My family had similar dynamics with regards to favouritism. For example, they did and have done very little to celebrate my birthday (no cake, no presents were normal) while my brother got designer clothes, motorbikes as a young kid and new cars as an independent, married, well-paid adult. It was done secretly, and whenever I found out about it, it was minimised (oh, no, it wasn't that expensive) or I was gaslighted and told I was imagining things and being dramatic.
Every few years outside of my birthday, I would receive something extremely expensive, such as a 2000 dollar musical instrument, that I had not needed or requested, and then made to feel guilty about how much it had cost. For years. I wonder now if this was their way of addressing the inequality, of assuaging the guilt.
As an adult, this transferred to our children. I addressed it once more and my mum reacted the same way as when I was a child. That I was imagining things and I was always so dramatic. The things is, I could see she knew I knew, but she still denied and turned it back on me. Still as hurtful as when I was a child.
I knew it would keep repeating and so we have very little contact. We also live overseas, so that helps.