When Fabian and Gideon Prewett first found out they were going to be Uncles, they were confused. Baby Molly and Arthur, having a baby? In the middle of wartime? Surely that couldn't be possible - after all, all their little sister and her boyfriend did was hold hands. Anyway, they had no idea how to handle a child! What do you even talk about to a newborn anyway? Politics? Quidditch? Whilst the child would obviously be cute (it was 50% Prewett after all), they decided their place as the cool uncles would have to be put on the back-burner for a while, provided Molly was actually telling the truth and wasn't just pulling their legs.
When Bellatrix Lestrange found out she was going to be an aunt, she was disgusted. Her blood traitor sister, marrying a muggle born? Having his filthy half-blood spawn? She wouldn't be having anything to do with that. And so from that moment on, her sister ceased to exist. Never again would the name Andromeda leave her lips, never again would she allow herself to even think of her disgraced sister. In fact, as far as she was concerned, she only had one sister, thank you very much.
When Andromeda Tonks first found out she was going to be an Aunty, she was regretful. She had always assumed she would look forward to this moment, this chance to be a role model for that tiny toddler as it grew. But she knew her less than hospitable sister would allow her to play no part in the upbringing of her nephew. However, whilst the thought of that child left completely in the hands of her chauvinistic relatives slightly terrified her, she knew she wouldn't change it for the world. She had a new family now, anyway.
When Sirius Black first found out he was going to be an Uncle, he was worried. Voldemort wasn't getting any weaker, he knew, and bringing a child into the world seemed a terribly large risk, particularly with the whispers circulating in the Order concerning a child born in July. Anyway, Sirius didn't know how to be an Uncle - it wasn't like he'd had any particularly inspiring parental figures growing up. Overall, he figured the whole situation was a bit too panic-inducing for his too-cool-for-school lifestyle.
When Petunia Dursley first found out she was going to be an Aunty, she was indifferent. It didn't matter to her, whether her freak sister and her weirdo husband were expecting, it wasn't like she would have anything to do with the child, who would undoubtedly carry her sister's freak genes. Anyway, she was far too occupied with the humongous bump of her own child to spare more than a thought to her sister's, for she knew that she, Petunia, would never let her child feel second best and didn't see much point in not starting now.
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When Ron Weasley first found out he was going to be an Uncle, he was ecstatic. Whilst he was fully aware that he was, in fact, already an Uncle (so there, Bill) this time he was THE Uncle. His second and third favourite people were making a baby that was half of each of them, and whilst privately, he thought that that was probably far too much sass to pass onto one child, he knew that the little guy would always grow up in a happy home. After all, the war was over, and he loved babies - he wasn't his mother's son for nothing. In fact, the mere thought was already giving him baby fever - hmmm, being a father can't be that hard, right? Maybe he should have a chat with Hermione...