I don't think it's that big of a deal if it's a name you love and doesn't sound too similar, or lend itself to the same nickname. So with Ella I would stay away from Elliot and Estelle because they both have that strong 'el' sound. But I could see Ella and Evie because they have totally different sounds
I don't like Eliot for a girl, with this spelling, and with sister Ella. Way too much.
Estelle is better, but Ella and Estelle? Too much of the Elle sound which I like, but not all these Elle names together.
Evie is better especially if you pronounce it like Eve and not Evelyn.
Euan is really cool (I like it with the Ewan spelling.) It could definitely work. So could Evan and Ethan.
So in general your boys names are really cool and would go nicely with Ella (particularly Euan). I would work on the girls names. Is there a reason you only like E names?
Here are some great non E names:
Ava, Vivian nn Vivi, Molly, Abigail nn Abby, Collette, Gillian/Jillian, Hayley, Holly, Isla, Juliet, Calliope, Lillian nn Lilly, Phillipa nn Pippa, Violet, Octavia, and Tallulah nn Tally or Lulu.
It really doesn't bother me if siblings have the same first initial. My two share the first letter "J", and oddly enough, it didn't even occur to me to be concerned until someone said something about it. I just used two names I liked!
If it's just two, I barely notice. If it gets to three, four or more...it seems soooo weird to me. Especially if you and your s/o share that initial.
I come from a family with a lot of C/K names and my parents were always calling us the wrong name. I still get referred to by the wrong name by people who know my entire family. If you don't mind the idea of possibly getting your kids' names mixed up for the rest of their lives, then I don't see any problem with it.