Should I confront my fathers girlfriend?

My father's live-in-girlfriend has never made an effort to get along with my sister or myself. My sister is 22 and lives with her fiance; I still live with my dad. They have been together for 4 years and my sister and I have made many attempts to include her in things, from conversation, to shopping, to trips to the gym, to manicures to lunch dates and she never accepts any invitations nor does she talk to us. I know it may be intimidating because we are both our father's little princesses and she may view us as competition but we have done nothing but be nice to her. Last night, my sister announced her engagement and we went to dinner. During dessert, my father was reminiscing about different things from our childhood (it was a sentimental occasion) and his girlfriend said, quite rudely, that she was bored because our conversation had nothing to do with her and she wanted to leave. She ruined my sister's engagement dinner, and started swearing in front of my ten year old brother.