Excess of every thing is bad. As you have certain time for eating, school, sleeping, playing & for study. If you give more time to one thing the other work will suffer .
It is not fair that you did not give compute to your sister.
She has aright to play as well .
You make a time table for your 24 hours. & write down how much time you will every work .
Then promise to your heart that Inshallah you will act upon it.
This is the way to build your character, hard work & discipline is the key to success.