Your Joel is still very young and its normal for them to play rough. Your little one Roxy is just doing what come natural for litter mate play. Since Joel is now her fur family !
I agree she isn't learning anything by being placed in her pen but sometimes it is necessary. I would suggest leash training (umbilical cord training) which gives you assess to her every move so that when she is playing to rouph or nipping and biting you can nip it in the bud...LOl pun intended !!
Be sure to use a harness with this type of training. You want to say with a very firm deep voice "
Ah Ah & NO" or "
ROXY NO" 'NO BITE" putting stress on the work
"NO" than follow up with "
Be Gentle". The key to stopping it is to try and stop her from ever reaching that threshold when the rough play and biting begins. If voice commands fail to get her to stop than you may want to try a Shaker can to get her attention or a time out as a last resort.
So , How does Joel react when she is playing rough ???