How do I fix my sister's wifi?

I’m visiting my sister and she has the worst wifi. Besides it being very slow, it looks like no two devices can be on it at the same time. So, if I’m on my Macbook and she gets on her laptop, the wifi kicks me out and says that another computer is using my IP address. Can someone help me out on how to fix this? A step by step guide would be much appreciated. I can try to find further information on the router, etc. if you need it. Thanks!