I have been with my now husband, dated for 12 years finally married 5 years ago. 
He has 2 daughters, 28 and 26. 
Soon after we moved in together the his girls moved in with us, mom is very unstable. They lived with us for 10 + years. The older SD and I had a very good  relationship since her and her mother did not. 
The younger SD has been a problem since we met, the only way to describe her is she has a dark side. She was a daddy's girl and I understand that she felt pushed aside, ( she wasn't) but she felt that.