I went only by ear, translated it as "She is your horse" and was going to make a comment about how kinky that sounds until I noticed the error.
How to differentiate between their guest vs your guest?
Probably, context. Both are correct translations, I think.
Now, deres means their?
It means "their" as well as "your" (2nd person plural). Depending on the context.
En av de beste sangene!