Take the deal dude... I'm sure in the back of your mind you don't want her to just have her weekends free to do whatever she wants, but the reality is that it's easy enough to find a sitter when you want to go out (or you can ask her to watch him), and there is a lot of weekend time when she won't be doing something and she'll be sitting in an empty house.
As for you, you will get a lot of quality time with your son. you won't have to rush through dinner, homework, and bath after school and not get the quality time.
M - 9 1/2 years 5/5/16 - Bomb drop - 3 week EA 10/31/16 - We sold house 01/10/18 - D Finalized