Granted, I'm sure expectant mothers of the past thought about this as well, I just don't think they were terribly concerned, enough to go around asking family and friends if Homer would make a good sibling name for Julie. I also don't think they thought about popularity they way we do now. It used to mean a popular name was one of the most well liked names, and now some future parents cringe at the idea of their child being known as "the other Jack" or "Jack B." (Not that I blame them, I'm in that camp too.) Thanks to technology, a healthy supply of books and blogs and websites, baby namers are very well informed. We are enlightened, educated namers. With lots more to worry about.
I'm sure many other blogs and websites have already written about this topic. This post, namely: http://swistlebabynames.blogspot.com/2011/08/baby-naming-advice-for-first-time.html and this post: http://nameberry.com/blog/how-to-choose-sibling-names. Many sites, such as Nameberry, will even help you figure out what name "style" you are drawn to. They often come up with new styles never talked about before. Maybe you like aristocratic names, or cowboy names, or nature names, or maybe several different styles appeal to you. No one ever said you couldn't have a boy named Wyatt and a girl named Willow. But you can't have a Willow and a Willoughby. (Why? Because they're too similar in sound and they can't have the same nickname.) Therefore, I won't even try to give advice on the subject, I'll just give you my two cents and a roundup of all the links I can find related to sibling naming.
Remember that these are just ideas to conisder, not rules. Ultimately, the way sibling names work with each other is completely up to you, based on what you think sounds good. After my advice I will include links to other pages that discuss the same idea.
My two cents:
Create a short list of names before you choose baby #1's name, and try to plan ahead for what you might name baby #2 and #3 and so on. By doing this you will be able to figure out your "style" and potential future names, assessing how well each name goes with the next. If you have a list of names that you can't imagine deciding between, that's good, because you'll have names in reserve for next time.
Get annonymous advice. Take advantage of Nameberry's message boards or Behind the Name's polls to see if other people think name A and name B go well together.
Don't name one baby after a meaningful (to you) place or person and neglect to name the next baby after something meaningful as well. This has the potential to lead to hurt feelings.
Here are some informative related links:
Nameberry's sibling naming advice:
Nameberry's naming rules:
Nameberry's general naming advice: