I think that time when the sister talked with Yu about Mame being her new mother, she may have just wanted Yu to confront her true feelings. Yu as all wishy washy before the question was asked. But with the outcome the sister now has started to act like...ummm.. like a sister should? I guess.. But we don't if she has some hidden motive.
It has been so long I can't remember WHO I was shipping with WHOM!!!!