Do "i'm with my sister in her hospital room" and "i'm beside my sister in her hospital room" sound naturally?If you need/want to explain, I agree with Hermione. However, "I'm with my sister" will do as a brief account. You can add, "in her hospital room", or '[I am] beside her in her hospital room' if you choose.
Thank you!Yes, they sound natural. As I said, the amount of detail is up to you. If you're talking to your mom it's one thing. If you're talking to your plumber, perhaps you want the details to be private. Where 'friend' fits, you decide.