The older Baldwin has been attacked by family groups after calling his 11-year-old daughter Ireland a "rude, thoughtless little pig" during a recorded telephone rage.
The tape of the rant found its way into the hands of TMZ.com editors, who broadcast it over the Internet.
But brother William fears critics are weighing in too heavily on a guy who is simply struggling with child custody issues.
He tells news show Extra, "Imagine you have been going through this custody battle for seven years. This is my brother's only child and you have this court-ordered phone call.
"It consistently becomes a pattern for four, five, six days (that he could not get his daughter on the phone)... I'm sure this feeds into the insecurities that my brother has that his ex-wife (Kim Basinger) is trying to poison the well."
He adds, "When I've seen my brother with Ireland, he's done nothing but foster and promote a happy, loving relationship, not only between him and his daughter, but between his daughter and Kim... My brother is a wonderful guy and a wonderful father."
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