I just reread what I wrote, this manga does weird things to me...
Don't worry... happens to everyone eventually.
By the way, Sumire is not a failure. She's an excellent Onee-chan by that worlds standards.
You need to keep in mind how she feels - that she still wants to grant and prioritizes Sou's wishes is a huge plus. Remember, it was Sumire who made Sou's school life possible. The other two would have flat out denied it and locked him up. Sumire is also the one who guards him the most outside, so she does a lot for him. Despite her flaws, she's quite a solid choice.
Sumire is older and has the required skills to handle pests (Karate), she's caring in her own way, quite thoughtful and reflects on mistakes. And yes, she certainly has loose screws but her antics are fun to watch - not just that, her looks are top notch. Finally, she doesn't hide her bad habits, it's "what you see is what you get".
As Sou I wouldn't touch her (she's blood related), but apart from that? She'd be on the list for sure. Right after Kana (finances, she has the background to manage a large household) and Yuri (food is essential, Yuri loves cooking and baking)... Sumire in combination with the Konno twins would be my personal guard team leader.