I can certainly understand why you feel you have gotten the short end of the stick. It sounds like you have not felt supported, and yet have done your best to accommodate your stepchildren.
I think there are two things that might help. First, I would encourage you to have a much-needed conversation with your husband about what you may need from him. It sounds like there needs to be more coordination and joint thinking about the ongoing issues. With such pressing needs making ongoing plans for care is essential. Rather than seeing each situation as a crisis — it may be better to plan for a more organized and systematic response.
Secondly, it may be time to look into other services for help during the week. Making connections with the Epilepsy Society in your country as well as the Autism Society can give you access to resources and perhaps volunteers who could help.
I believe these two things may start to offer some ways of providing relief.