I will give as brief a summary of the situation as I can, as it is complex to say the least.  

DH and I have been together almost 7 years.  DH was with BM for 25 years starting in high school.  Children of their marriage are (to me) SD25, SS21 and SD15.  All SKIDS live out of state with BM, who was an adulteress and is currently remarried.  Both BM and her husband are alcoholics and narcissists (by all accounts).  DH also has ExGF that has glommed onto BM and the kids.