Is it true that parents favour one child over another? Does this mean that their parents love them less?

I am the only child and a late child to my parents. 12 yrs post their marriage :) My mum was and still is very close to my eldest cousin who was brought up by my mum. In fact when they were trying to conceive, mum used to take tare of her as her child. Even now sometimes,mum calls me Ruky sometimes by mistake instead of Nanu (my nickname). As a child I did feel mum loved her more than me-even though she was a cousin. But now that some maturity has struck, I am able to see that yes, Mum does have and will always have a special bond with Ruky. But I know that her love for me is endless. As her child I get her love and prioritization always. I feel the same would be with siblings. Love would be unconditional for both but one of the siblings would always be mama's or dada's pet !