I think it's obvious, you tell your partner that until little Susie starts taking her meds as prescribed, you don't want her there whilst he's away. She can stay with her mum.
When he gets defensive about it remind him that she's violent, and you have the welfare of the little one to think of as well.
Schizophrenia isn't the horrible murdering arsehole people make it out to be. With proper medication it's very manageable. However, without the meds, psychosis is very real and absolutely a possibility. It's not fair to put so much pressure on you.

Remove yourself and your little one from that dangerous environment.

My friend's stepson was in an inpatient program until he was med compliant and at therapeutic levels. She could become quite dangerous to your other children.

My sister has schizophrenia and even when she would take her medication it took many years to become manageable. Often the medicine would work for a while and then stop working and need frequent swapping or have terrible side effects. She had many times when she ended up getting violent.

If it gets too much you may need to consider temporarily splitting from your hubby until her condition has improved. Even if it means he quits his job/she lives with her mum for a while. Sometimes episodes may be dangerous for your younger child and your step daughter may need more individual attention while she is getting used to things. Not forever but just until she has settled and found the right medicine.

Otherwise if she stays with you than lock up all knives, razor blades, scissors etc in your house. In an episode she may try to harm herself or others with them especially if off her meds or as a side effect to different newly prescribed medicine. Consider using plastic cutlery and locking up all sharps until her condition is properly managed.

Take her to a Psychiatrist to focus on why she isn't taking her meds and to monitor her reaction to the meds.

Check if she is getting triggered by anything when she is screaming/violent. TV and radio would trigger my sisters episodes as she was hearing voices we couldn't from devices...

Consider a pet. We found an overly affectionate cat that loved being patted calmed my sister faster. The cat learnt to start meowing before she would have an episode to help alert us.

Also look up The B Team on Facebook, may be something useful on there about helping her overcome her concerns about the medication.