Babysitting friends almost 3 year old

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My husband and I are expecting our first child and we have baby sat my friends son a few times already but her and her kids dad are in the middle of a divorce and he hasn't seen his dad in probably 2 years and he just learned to say "daddy" is it wrong that I get jealous that he calls my husband daddy? I feel bad for feeling like the only one calling him dad should be our son when he gets here in September, am I being selfish? I feel silly but so jealous at the same time