Ryder Confirmed to be Brother/Sister

Exactly. I can easily see them restricting your responses to matching the canon version of that Ryder. For example if male Ryder is more lighthearted, them restricting you to lighthearted options in dialogue. Or if the female Ryder is more hopeful, then her only having hopeful idealistic responses. Also, what if they have different backgrounds? If you want to play a certain background, you may have to be only male or female. The list goes on. 

Tbh, I already figured that the N7 guy might be related to us in some way, such as our father. I unwittingly spoiled Fallout 4 for myself in the same way, simply by imagining the most messed up scenario I could think of, which ended up being the exact reveal the game gave us.

I need to stop speculating so much, I keep ruining games, films and television by working out the plot twists too quickly.

It doesn't bother me since there's a lot of stuff in the game that I will discover without been spoiled.

Bioware would have probably announced such a bombshell revelation long before now, but what if it turns out that we could play as both Ryder siblings at various points in the game?

It'd remove the issue of people feeling constrained to following the "canon" personality established for the sibling we didn't pick, since if we are playing as both of them, we are effectively in charge of creating both their individual personalities?

Bioware have talked about messing around with two protagonists for a while, so having them test the water with this feature in Andromeda might be something they'd consider, especially since the whole point of the series moving Andromeda is to give them more freedom and less constraints with how they tell the story.

Real talk: where/when has Bioware said anything about a dual protag for any of their games?

I could have seen that working with Hawke and DAI at least. If I wrote it, I would have started as the Inquisitor, same as the game, who had the mark, but wasn't exactly leading. That'd be Hawke. And I'd combine Haven's showdown with something like Here Lies the Abyss. Maybe Hawke fails in that and your main character took lead. Although you wouldn't necessarily have to control Hawke ever (much like DAI is already).

It's almost like the anime Gurren Laggan, if you've seen that.

Good shit

I don't think there'll be a multiple protagonist approach based on how Walters explained the family thing. He said we'd pick one and the other will be somewhere around.

I'm not honestly worried about having problems in going against the 'canon' sibling in terms of personality. I'd criticize a lot if they limit the choices in dialogues based on a preset personality, but I don't think they went that route.

Only way I could see a dual protagonist work, which I've stated in this thread (I think anyway, coulda been another thread?) I generally think it's a bad idea and unlikely to happen in MEA. But, thinking about how it might work in a manner that isn't horrible... I'd say it would have to be 2 separate stories that play side by side instead of 2 people same area swap at will kinda thing.

Kinda like Halo 2, or Halo 5 (only more like Halo 2 cause 5 had the chief like twice). Where the narrative juggles who you are every chapter. Each person would have to have there own squad, and companions. Or every other Chapter is the 2nd person and it's an 'in the future' thing looking at past events.

If DA2 had done that there would be a playable part between each 'act' as Varric or Cassandra (probably Cassandra) where you searched for evidence and interrogated Varric constantly through out instead of the cutscenes we had before. THAT could be fun/interesting and wouldn't of detracted from 'Hawke is us' feeling.

Which is, I think people have pointed out, how Witcher 3 does it. To some small extent how ME2 did it with Joker which was freakin' amazing sequence. Past that and your just going to be created piles of extra work for added confusion and would heavily dilute the players connection to the perceived main character.

The family is the most exciting aspect from this new ME imo

I'm going to stick my neck out and say that the ME:Initiation book is going to centre around papa n7.

I wonder if Ryder will see some distrust from colleagues. He's/she's inexperienced after all. What if we're only in Andromeda because daddy pulled some strings? What we've seen from marketing is that the Andromeda expedition is recruiting soldiers of the highest quality(be it Krogan or N7). I wonder what they'll think of Ryder who's still a bit wet behind the ears being trusted with leadership?

But you are not going to play one of them.
You either will be playing a male character who will happen to have a sister with x personality OR a female character who will happen to have a brother with y personality.
The personality of your NPC siblings won't have any influence on the personality of your character, because he/she will be a different person.
It's the same things as Hawke, with the difference that this time you won't have both of your siblings at the beginning of the story.