Ok so I did some digging and in a way I wish I hadn't but here goes.
Let me start off by asking are the Ice Climbers siblings, lovers, or just really good friends?
Nobody really knows for sure, and Nintendo's official statement on it has been it is up to our interpretation.
In the smash series however, there have been two instances suggesting what Sakurai's interpretation of their relationship is.
As you can see from this picture, they share many physical attributes, not to mention they sound a like (same voice actor), it wouldn't be out of the question to think that they are twins on first glance.
During Snake's codec call with Meiling, things get interesting.
Snake: So those two are the Ice Climbers, huh?
Mei Ling: Snake, have you ever heard of a "blood bond"?
Snake: Sure.
Mei Ling: It refers to a bond between two people that's so strong they'd die for each other. The Ice Climbers have conquered frozen glaciers and dangerous mountaintops together. I think they've formed a bond that we can't even fathom.
Snake: I dunno. Seems to me they'd get in each other's way fighting to see who gets to the top first.
Mei Ling: Snake, in China they say, "Though brothers may quarrel at home, outside they defend each other from slight."
Snake: Is that really a Chinese proverb?
Mei Ling: Even though they may get in each other's way occasionally, they'll pull together in times of need. They're an inseparable team, Snake. Don't underestimate them!
Source
This proverb is typically used to describe family members. Just based on this it would seem Sakurai's interpretation is that they are siblings, and it makes sense.
However it doesn't stop there.
While there have been numerous hints that they are siblings, there is one piece of media that doesn't just hint, it exclaims that they are lovers.
This shot is seen 1:17 seconds into the melee intro.
With that I don't know what to think, perhaps they are both siblings and lovers?
Technically we are all related and this was a common thing in the past. I'd recommend watching this video for more information
[12/4/2012 2:48:31 AM] CoF DeLux: maybe
[12/4/2012 2:48:37 AM] CoF DeLux: they are both lovers and siblings
[12/4/2012 2:48:46 AM] CoF DeLux: which would explain why they are ********
has spoken.
