linksys wireless router

i have a problem with my linksys wireless router. when i use my laptop to connect to the router, it is working fine, but when my sister tries to connect to the internet in our personal computer while i’m using the internet in my laptop, there is no connection of internet in our pc. after a few minutes, there will be a connection again, but it keeps on disconnecting while my sister’s using it. this only happens every time we are both using the internet using my laptop and her pc, help pls.