I love the suggestion of [name_f]Shoshana[/name_f]. [name_u]Sasha[/name_u] sounds like a very logical nickname for it. I also like the meaning “lily.”
I like [name_f]Saskia[/name_f] if you don’t use [name_u]Sasha[/name_u] on its own. [name_f]Stacia[/name_f] is another possibilty. [name_u]Evelyn[/name_u] and [name_f]Evy[/name_f] are lovely, too.
I think [name_u]Evelyn[/name_u] [name_u]Sasha[/name_u] would be a good choice for you all. To my ear, [name_u]Alex[/name_u] and [name_u]Sasha[/name_u] just don’t sound very good together, even without them being nicknames for the same name.
If you really want to use it, though, I think [name_f]Susannah[/name_f] is a good option for a longer name.
i think charlotte would be perfect. evelyn charlotte or charlotte evelyn. then definitely call her sasha. no one will notice most probably! buuuut I think it would still be nice for sasha to have a formal name, just like alex has alexander. if he was just alex i would say just name her sasha, and then you have both nicknames for alexander, which is cute. but he gets the full name, so sasha should get a full name as well 
i vote alexander and charlotte (sasha).
Maybe something like [name_f]Sarah[/name_f] [name_f]Charlotte[/name_f] nicknamed [name_u]Sasha[/name_u]? To me it works, taking the ‘Sa’ from [name_f]Sarah[/name_f] and the ‘cha’ from [name_f]Charlotte[/name_f] to form [name_u]Sasha[/name_u]. But I also think [name_u]Evelyn[/name_u] is a great option as well, and I love [name_f]Evy[/name_f].
I really like the suggestions of [name_f]Sarah[/name_f] [name_f]Charlotte[/name_f], [name_f]Anastasia[/name_f], or [name_f]Cassandra[/name_f] w/a nn of [name_u]Sasha[/name_u]. I wouldn’t go w/[name_f]Alexandra[/name_f]…although I do have a friend named [name_m]Victor[/name_m] who named his children [name_m]Victor[/name_m] & [name_f]Victoria[/name_f].