It's not using 49% of the battery capacity, it's using 49% of the amount of power being used. If you have your screen on for a longer percentage of time, you'll see the screen usage go up in percent and the OS go down in percent.
The percentages always add up to 100%. IOW, 100% of the amount of power being used is being used - even if that's only 2% of the battery capacity.
Evidently your mother is using something else more than you are - like her phone or her screen.
The OS will always be using battery, even when the phone is in deep sleep - otherwise it would never come out of sleep.
Tips for conserving battery include keeping the screen brightness as low as you an use it comfortably and keeping everything you're not using - data, GPS, wifi, Bluetooth - off until you need them.
Also, condition the battery. Charge it, turned off, until the phone indicates 100% charge. Unplug it from the charger and don't charge it again until the phone tells you to. Do that 3 times. Then, for maximum battery life, try to charge it when it gets down to 40%-60% charge.