Is there a word for your parent's significant other's children?

A version of the popular phrase, "My brother from another mother" comes to mind. How about "My brother from my mother's significant other? or the shorter "Siblings from mom's (or dad's) Significant Other."

There's the term "prospective step-siblings", which implies a significant possibility that the parents will get married. You could also precede "step-siblings" with a variety of modifiers, such as "quasi", "pseudo", "para", "almost", etc. You could also apply modifiers directly to "siblings", either the above, or another such as "meta", "allo", etc.