Q: on my cousins Apple music family plan and I'm wondering if I'll b charged for downloading or adding music to my library

Hello juniornguyen,

Thanks for reaching out to the Apple Support Community. Apple Music's family memberships are fantastic for accessing music and sharing with your family. The good news is, so long as you are not purchasing the content, neither you nor your cousin will be charged. Adding the content to your music or downloading a track is included in the monthly membership cost.

Just as a heads up though, if you are removed from the family plan, or if your cousin cancels their membership, you will no longer have access to the songs you added or downloaded. 

For more information see this article, it will help you with any basic questions you may have on Apple Music and family memberships: Apple Music and Family Sharing

If you have any questions or concerns, the whole community is here to help.

Best Regards