We a kind-of-discussed this question in Chapter discussion thread, but that little dialog didn't satisfy me. I think that this question deserves separeted thread so here I am.
Actually, the question is: what is going on inside her head?
From past few chapters it's pretty obvious that Yakumo is now supporting Harima's desire to get Tenma. It doesn't matter if she has restrained her own love to Harima or she just tries to help him as friend (in case she never loved him). The fact is: Yakumo is helping Harima not paying any attention to Tenma's feelings. Thats very strange (at least for me) and goes complitely out of my imagination of Yakumo's character. If earlier her main affair in life was to help Tenma - now it is complitely different. When does her feelings changed so much that she now pays zero attention to Tenma's feelings?
I do think that she care less about Tenma then about Harima basing on this info:
1) That Harima loves Tenma.
If it was the only info that she has it would be natural for her to try to help her friend, but she also knows
2) That Tenma loves Karasuma.
So WTF? It must be obvious for her that only happiness possible for Tenma is getting Karasuma. But she, instead of helping her sister with this deal, helps her friend, who was till nowdays less important to her. WHY?