I think Yakumo wasn;t really aware of the things she did.
Let's face some facts :
1) Yakumo wants her sister happy
2) Yakumo wants Harima be happy
3) Yakumo hates Eri - although she will never admit that
4) Yakumo loves or at least likes Harima - although she won' admit it either
So let's think like Yakumo
"Nee-san wants to be with Karasuma-kun but she does like Harima-san in some way, so she won't mind spending some time with him"
"Harima-san wants to be with Nee-san"
"Sawachika-senpai......She doesn't understand the situation..."
"I can't interfer - I will just be a nuisance for Harima-san and Nee-san"
So she just arranged the meeting with Tenma and Harima - looks pretty harmless - they will meet up, it's all about if Harima will use the situation or not - for Tenma it was fun to help Harima so there wasn't really problem.
And that's it - although she doesn't like Eri, she never thought about making her miserable and never thought of her interference. She might just feel a little responsible for what just happened, but I don't know if she should - It would happen later or sooner.
As for the chocolate issue... we know that Hanai's feeling aren't love but obsession - he doesn't know anything about Yakumo. That's why he ate the chocolate from the girl she makes fun of him and hadn't ate the chocolate he presumably "love". Hanai is just an Otacu going around the seiyuu idol - a really immature feeling.
Sara isn't at fault, as I say, it would happen sooner or later