There is nothing abnormal in your response to this incident. You haven't seen the guy in over 10 years, and even back then, he was your brother's friend, not yours, so you didn't really have much to do with him back then, either. There is no reason why you should have any real feelings for someone who was the next thing to a stranger. Your feelings for a family pet are a different matter. The pet is someone you spend time with every day, who is actually a member of the family. Of course you would feel something about the lose of a pet.
The prober thing to do is to say the right things, whether you have feelings to go with them or not. Just say you are sorry to hear that, and are sorry for what the family is going through. That's what I do when I don't have feelings to go with similar situations. Although I seldom have full feelings for other people's situations, I can usually understand what they are going through, and can have abstract empathy feelings for them. That is not the same as a full feeling of empathy, but it does help me relate to people.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau