Should The Twin Be Recruited as a Squadmate in Upcoming DLC?

Definitely.

I also hope we occasionally switch control between them. The one time we did in MEA was one of my favorite bits.

It could be interesting especially since it wouldn't be viable to make them a squadmate on a regular basis.

I don't see why not.  They would be a squad mate like anyone else in the squad.

Personally I love the idea.  I think there is a lot of room there to make a more realistic feeling squad.  They could use data from your saves to setup aspects of the game.  Say you play as Scott and romance Cora then in a NG+ or the sequel you can choose to play as Sara and you get to see Scott and Cora still in that relationship (obviously would have to account for bi characters in some way) or in DLC and a sequel you would get to see choices Scott made play out in your Sara play through.

It would take some thought and planning on BW's part but they could do something really cool with having both siblings in the game.

I wouldn't mind but keeping them as a side character is fine with me.

What if they both romance PeeBee or one just had the one night with her in the pod? Imagine how awkward that would be. 

While I'd love if Sara could join the Pathfinder team as a regular squadmate, I don't think it's going to happen. BioWare would have to add a lot more dedicated dialogue for both Ryder siblings. Not only would both voice actors have to do dialogue for the Pathfinder, but then they'd also have to do dialogue as a squadmate.

No, because their playability leaves their personality and powers too malleable and ill-suited for an NPC with major presence. Sara and Scott have some defined backstory, but they are not Bethany and Carver.

Personally I'd rather see the sibling get a team of their own. Of course, we might have people going "Aww man s/he has better squadmates! I hope their ship explodes!"

Sure.

It would be great if they make fun of each other. Scott tells the crew and squad how Sara did this, that and the other things as a kid.

If they could pull off the multiple sets of writing then yeah I think it would be very cool.

Yes! I've wanted that option since I knew about the game. I love family moments.

No, make them governor of Meridian or something.