First, I commend you for being a sister-mother. You were very brave and selfless to answer that call.
I have 5 children. My first 2 were born a year apart and so were my last 2. If you are looking to do something that will totally stop your daughter from showing any signs of jealousy, forget it. Don't be so hard on yourself. Competitiveness is natural to humans and it doesn't mean that you are not doing a good job of tending to the needs of either child. Siblings will show some kind of jealousy toward one another for the rest of their lives.
For myself, I try to focus more on making sure that I have a unique and special bond with each child. For my oldest daughter (13) it's nail art. That's what we do. One of my sons loves music so I spend time sharing different artists with him and he shares some of his with me. My other son loves cars so when we take drives we always keep a lookout for classic cars and talk about them and the paint jobs, chrome, etc. Find something for each girl that can be "your thing" together. You will drive yourself crazy trying to eradicate signs of jealousy. I made that mistake as a young mother.