Why does my dog bark at specific person ?

I have a 5 year old male chihuahua and he seems to be a normal dog . When new faces come to our house he'll bark at them first then stops and gets comfortable with them . But my uncle and his wife and 2 kids always come almost every other day . But my dog is STILL very uncomfortable and continues to bark loudly at my uncle loudly . He'll growl/bark at everything he does . One time his brother came to the house with him and they're both bald to be specific , and my dog got really scared and refused to come out under the bed for the whole day . It was like he was defenseless . But still he barks at him constantly . My uncle tries to be friendly but that makes the dog more aggravated , What's my dog's problem ???