I would like the idea. Absolutely. Out of likes but yes, I love this.
1) For me, the best way to approach this without making a narrative mess is making PC Ryder your protagonist during all game. However, at some points of the gameplay, for brief periods of time (maybe 5-10 minutes of narrative and some combat) you play as the N7 Ryder, the antagonist. If the N7 is a sibling, the opportunity of playing with him/her would be a great way to make us feel a connection. And a very good way to make us care and make us feel sorry for PCRyder about what that sibling is doing. Also, it could be interesting to play for the opposite interests of your protagonist time to time (maybe I'm wrong but The Witcher 3 already did that, didn't they? Geralt is the main protagonist but his surrogate daughter is a playable character time to time).
Also, it would make us conflict if in the end you have to make a decision like let your sibling die for his/her crimes or save him/her with the cost the sibling escapes and everyone hates you.
2) Two siblings, brother and sister would be great. If the N7 sibling is the older, it would be a great dinamic to oppose him as a less experienced and younger Ryder who had grown up idolizing the figure of her brother. The opposite dynamic is a great as well: if you play as maleRyder and then N7 is your sister, it would make the unusual approach of a younger and less experienced Ryder who had grown up idolizing the figure of his older sister.
3) The game uses the two Voice Actors regardless of who you choose as the main character.