Currently, Yakumo doesn't know the story of what happened between Eri and Tenma. As such, she isn't in a position to judge who was right and who was wrong. Any assumptions made in this manner would only show a bias towards her sister and/or a bias against Eri.
Furthermore, Eri hasn't actually done anything to hurt Tenma; rather, Tenma has been hurting Eri. The only thing that Eri has done so far is to avoid Tenma - and everyone has a right to get some space.
Ch.206 shows Yakumo developing somewhat of a fangirl personality towards Harima - she doesn't want him for herself, but she expects him to behave in exactly the way she imagines him to be. Her interest in pairing up Tenma and Harima is solely because she doesn't want him to change. That's part of the reason why the possibility of an Eri x Harima pairing is so frightening to her - if Harima switches his love interest, he cannot live up to the image of being stubborn and steadfast.
Of course, the segment with Iori at the end of the chapter serves as the antithesis to Yakumo's viewpoint: no matter how much Yakumo wishes for Harima to remain the way he is, Harima can and will continue to change. No matter how much Yakumo tries to force a pairing between Tenma and Harima, nature alone will decide the ultimate course of events.
I wouldn't be surprised to see open hatred on the part of Yakumo towards Eri. We've already seen her deliver some snide remarks to Eri in Ch.205, and she did have a fairly smug smile on her face when she separated Harima and Eri after taking over Iori's body in a recent b chapter. Such emnity isn't really all that surprising, given that Eri probably represents the biggest obstacle to Yakumo's vision of what Harima "should be", at the moment.
Tenma and Eri will need to resolve their conflict on their own. Eri's probably going to have to enlist Tenma's help if the two of them are going to win over their respective love interests - especially given the fact that Yakumo will be actively working against them.