So, if I understand you, your stepson's wife stabbed him at least twice. The injuries were serious enough to collapse a lung, and she went to jail but bailed out or was released after only 24 hours??? Now, she's actually coming to visit the man she tried to kill??
Have you spoken to the hospital staff about this? They CAN limit visitors if they opt to.
You or your stepson's mother needs to speak with the investigating agency and tell them this is happening. They can restrict visitation or even seek an order prohibiting contact. In many states (unsure on yours) an order of protection is automatic in such matters.