The perpetually offscreen brother
Just because your character has siblings doesn't mean those siblings are relevant. There's no reason to assume that your character in The Secret World had an ornate family history of mysticism, and it's quite possible that her brother is a middle manager in an office downtown. She calls him up on occasion and makes plans to get together on holidays, and there's not real impact on her day-to-day activities. They might not even like each other very much.
This carries a fairly obvious benefit insofar as it requires more or less no effort from you. If your character mentions a sibling or two, it can be thrown out occasionally as a bit of background and left alone. Perhaps that part of the family just isn't all that relevant to your character, or maybe it's just a case of very different lifestyles -- your Norn in Guild Wars 2 assaults dragons, and his sister is a florist. There aren't many opportunities for both people to connect in a meaningful way.
On the other hand, you are ensuring that you can never use those siblings for anything more than background information. Sometimes that's for the best (I have a character with five siblings who can't really be brought into play through anything other than background), but sometimes it seems like these characters should be more relevant. It certainly obviates any interesting sibling interactions. You wind up with set dressings rather than people you can talk to.
Oh, and you're more likely to forget names. Or at least I am, anyhow.
Meet my brother; I'll just vanish for a bit
So let's say that your character has a sibling who is relevant. Very relevant, even. Sure, your character is a scientist and her sister is part of the military, but both have a reason to wind up on Nexus in WildStar. Naturally, this is all part of your plot to have both characters under your control as alts, meaning that you can play both siblings as necessary for roleplaying and have all the benefits of lingering sibling arguments that have gone on for a decade just because one of you got the top bunk.
For those of us who play a lot of alts, this can make a lot of sense. Your characters are connected by something more than just the same account, and there's good reason for them to know one another. There's also a plausible reason for these characters to look similar, so if you just can't stop playing Draenei or whatever, you've got an excuse. Your characters all fit together into a meaningful whole. And let's be honest, there's a certain fun to having one of your characters serve up a whole lot of dirt on another character, especially to that other character's love interest.
On the other hand, this does raise the question of why Bruce Wayne and Batman are never in the same room. Your character is never going to be able to interact with a sibling on-screen, meaning that all of the dynamics are informed attributes rather than observed ones. Plus, there's the usual trouble with alts -- it might turn out that you really like a character's traits but don't necessarily enjoy playing the character proper.
Guest-starring someone else as my brother
If you've got a roleplaying partner you trust, sometimes the best option is to have that partner take on the sibling role. Now your character can be right there with his brother, and both of you can explore that dynamic in real-time. Maybe they're terrors when together, or maybe they don't get along, but either way people can observe that sibling relationship in action without any need to swap characters or the like.
The benefits to this are pretty obvious just from the premise. Both characters wind up with a clear and present sibling with defined traits and characteristics. You can develop that relationship and other relationships organically, with no need for any sort of character-swapping or the like.
Of course, this relies entirely upon someone who is both able and willing to do this. I've witnessed several people playing characters with siblings who weren't nearly as present or interested, or siblings that showed up, had a role, and then left without any more explanation. Not the best dynamic. Plus, you can't do this with just anyone. You need to have someone you can trust with a big part of your character.
Yes, I said your character. Siblings define one another, and you need to be prepared to work with another player to define the shared history of both characters. Even if the two were separated at birth or something similar, there's a story there and a corresponding set of expectations. Having someone else playing your sibling means relinquishing some control in favor of having that on-screen relationship -- to someone who may or may not turn out to be all that reliable.
So a bit like having actual siblings, then.
Feedback is welcome down below, as always, or via mail to firstname.lastname@example.org. Next week I'm going to chat about starting that first conversation with another character, and the week after that I'm going to talk about getting into the mindset that allows you to make stupid, shortsighted choices that are bad for your character and good for entertainment.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did. If you need a refresher, check out the Storyboard Library.