Please bear with me while I tell this story... I'm super frustrated and disappointed with my partner's kids, and generally exhausted from their BS. 
For context: they are three adult women, in their 30's. 
My partner and I have been together for 10 years. Their mother passed away about a year before he and I started dating. 
We are in a very happy, fulfilling, committed relationship, and plan to spend the rest of our lives together.