Maybe angle it more towards general advice, and maybe day it's worked for you? As you have children the same age, I'd probably say something like "My DD went through something similar. She was so used to having me to herself for a few years, it was hard when DS came along and as he got older she started acting out. I found spending good quality one on one time with her helped and was important to build our relationship together apart from DS."

There's no need for her to know your thoughts re favoritism, but it sounds like the girl does need some time with her mum.