We have twins (and 2 older kids). My husband definitely prefers one twin. It happens. I simply give the other more attention to balance it out. You should give the boy a little extra love and attention to balance it out.
It's not that big of a deal.

the mom needs to fix this, not you. just help build both children's confidence and independence. for sure for the semi 'neglected' one.

I know it must be tough to keep your mouth shut, but it would be in everyone's best interest.

The PP is correct.....No matter what, you cannot force a person to love their child more.

It is totally out of your hands unfortunately.
Hopefully w/the passage of time the father will change his ways.

Nothing that you will say will make him like his son. Difficult children are liked less period.

That's sad but it's probably not something you - or anyone -can change. Once they are older the boy will figure it out and have a worse relationship with his dad than he could have.

I nanny for twins. I started when they were 3 months old. The mom let if slip that DB prefers the one twin more because she's " easier". He would give her the comfy swing, pick her up quicker if she cried, feed her first over her brother, etc. Now that they are 15 months, it's worse. You can still see DB prefers the girl ( only because she slept better at night). She gets the first of everything. Yesterday they were playing and she wanted a toy he had. DB took it from him and gave it to her. He will cater to her needs first. They stay with him two days a week. It really concerns me. MB doesn't seem to care. I want to say something but I don't know what or how. He isn't neglectful or mean to his son but there is a clear preference.