Id say yes the dad and granpa treats the children equally although sometimes they show favorite towards the fl but it's because she's been abuse and got treated poorly by her mother's family but I wouldn't say the favoritism is but it's more like they're way of showing love towards the fl after all she's been through I suggest you read it for yourself it's very good ( there's times all the children hang out together while the grown ups watch it's very wholesome)
All of the children are treated fairly and with love. Like the comment above me said, Dad and Grandpa do have a little favoritism towards the MC but that stems from the fact they were under the impression she died right after birth and was severely abused before the family reunited with her.
One of my absolute favorite thing about this manhwa is that the children actually act like typical kids of their ages.
Someone please confirm that all kids are treated fairly and equally loved before I dive deeper into this. Thanks!