How should you treat an adopted son/daughter?

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I have been quietly observing how my cousin treats his son, an adopted child. I noticed the child was given little attention, while the daughter (biological) has the chance to hug his father, the adopted one only sneaks a peek at them. It's pretty obvious that the child wanted attention too, but the father keeps on ignoring (sort of) the kid. I wonder what I should do: should I correct the situation, or is it too nosy of me to meddle up with their situation? Additional information: The kid was a child of one of my relatives who gave him to its current family since they could not provide for the needs of another child as they already have many children to feed and clothe. His mom(not the biological mom) tries and somehow succeeds, to an extent, treating her children fairly. The dad is just the "unfair) one.