Q: What to do if you're listening to music then suddenly it stops and a pop up says "Looks like you're listening on another device. Click here and press play" so the music starts playing but soon stops and the same pop up comes to show, what do I do l?!

Taking a guess here but, do you share an apple ID with your sister? Apple Music only plays on one device at one time! (If you're using the same ID.)

I could be wrong but I feel pretty right.

If you share an Apple ID, you can play Apple Music and Radio tracks on only one device at a time unless you have an Apple Music Family Membership, which lets you play music on multiple devices. You'll need to either create a separate Apple ID for yourself or your sister or purchase a Family Membership.

Manage your Apple Music membership on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support