People with your SIL's issues have a really hard time speaking/asking/behaving directly. Because if they ask directly for something, and they're told no 1) it means they don't get what they want, 2) it means there is no "back-up plan" and 3) it means they have to face the reality of the "no" and all that implies in their head. i.e. "My brother said no to my unreasonable request, so my brother must not love me."
These personality types are masters at triangulation and playing people against each other because a vast majority of the time, miscommunication, misunderstandings, and the awkwardness of the miscommunications and misunderstandings get them what they want. And in the rare occurances in which the triangulation doesn't work, the manipulative personality can tell themselves, "Well, brother would have given me what I wanted, but mean old SIL got in the way!"