Our dd was 8 when we started the adoption process for Marisol. She had wanted a baby sister for a long time. She was the first one to actually say that we should adopt. We included her in the entire process, meetings with the agency, visit and pickup trips, getting things ready for Marisol at home. Marisol was 17 months when we brought her home. They love each other very much and there has been minimal conflict. They are now 28 months and 9.75 yo. Sometimes when Kara just needs to do her own thing, the little one doesn't understand why she can't have big sisters full attention.
I will step in and get Marisol interested in something else. But for the most part, Kara spends a lot of time with her sister and enjoys her very much.