i always would say to my friends "where there's a baby, there's a baby momma" meaning it's a possibility there will be drama.
you have to have a strong love to be in this relationship. you have to sit down with your finance and talk to him and have boundaries and rules.
i was in a relationship where i had to deal with a women and her two kids (which wasnt even my b/f's kids) long story.
but i felt like she was always there, and it was his fault b/c she was fisty and bossed him around.
i also can relate to him being angry and taking it out on you b/c he's pissed. understand this is his baggage not yours. you have to talk to him and figure out what yall can do about that crazy women.
my advice to you is do not get into a marriage if this isnt resolved because then you will fill traped, or regret that you married into such a stressful situation.