I went thru many yrs in your shoes. It is very hard to deal with stepchildren. they tend make Dad think stepmom is the wicked witch type. I ended up leaving mine but it was more due to being his punching bag. funny thing is now his daughter loves me. she even told me she was sorry for the way she behaved when I was raising them. they lived with us for yrs.
Only you can decide how much you can handle. talk to your husband and get him to back you up and let them know the adults are in control. good luck