I think Lydia and Marian are too formal for a sister for Scout. Not bad choices but I think something a little more unique and playful would work. Take a look at these names
I agree with Piper as a second name, and Piper Lydia for a combo!
I like the idea of sibsets that don't necessarily "match". I love both Marian and Lydia, with maybe a slight preference for Lydia. Scout and Lydia sound so cute together! I think it shows versatility and style as a parent to choose names that are in different categories.
Oh, from above -- I adore Hazel. Scout and Hazel sound super cute.
I agree with
I thought of:
Sage (Maybe a little matchy with Scout, but still good)
I think these all sound pretty good! I think with Scout you should stick to either unisex names or at least spunkier girls names (like Piper). Good luck! And I adore the name Scout.