Answer:
Keep them occupied
Step-by-step explanation:
Honestly I'm not the best person to be asked this though i do have a younger sibling but he's only 2 years younger so not much taking care of him happened. But keeping them occupied is probably the way, or depending on their age i guess, have them nap. Have them watch a movie they like and tell them that if they bother u you'll end the movie. And check up on them every so often.
Also I would like to add that if your still a child, high schooler counts, u shouldn't have to do this. You should tell ur parents that u shouldn't have to watch over them when ur busy studying, although I do know there are some tough situations. Do know u are not responsible for this, yes as a sibling u should take care of them, but if it taking care of them affects u, tell ur parents. In the end ur still a child/teen urself