What you have described is a problem that most families face - and it's actually quite healthy. Your 10 and 11 year olds have hit that pre-teen time. They are competing against each other for everything including your time and their elder sisters time. As a single parent, there are several things you can try. I would make a pact with the eldest daughter to work with you on this issue.

To quell that in fighting for attention, give them dedicated attention within boundaries. You may take one shopping while your eldest teaches the other make-up. The next day, you swap over. There does need to be conditions such as behavior, but you can work on that as you go. If both yourself and your eldest daughter can give them dedicated time, the competition between them will slow down, and you will actually have a stronger family - especially if you dedicate time to be together as a family.

As a footnote - don't forget to give your eldest some quality dedicated time as well.

Les