This kid scares me

My SO's kid came down here to live with us. She is 19 and pregnant. Never had a job in her life, didn't graduate school and is pregnant. Well I had no clue this was just the tip of the ice berg. This kid is only worried about boys, has no respect for family or kindness or anything. She is manipulative to the max, like I have never seen a kid prey on people's weakness and emotions like this kid does. I have two girls myself 17 and 21. My kids were brought up to have respect for elders and chip in and be a working part of society and the house. Now mind you I have very few rules. No drugs, no drinking and driving and text when you change location. My girls really never had chores just if I ask you to do something do it. My oldest just moved out to another state to attend college to become a psychologist and my youngest wants a career in makeup. His kid is used to getting anything she wants anytime, she lays around all day like a piece of garbage and texts boys. She got caught with weed a week ago. Was grounded, believe it or not my SO almost left me to defend that it wasn't hers. Well he realized the grocery bag it was in was from the state his kid moved from and not from here. Then he believed me. Well now spoiled little brat is creating nothing but issues so I want her gone. She is moving back to their home state in a week. My SO still blames or makes excuses for all of her behavior. Makes me feel like I'm the meanest person and all I want is to have decent little humans in the world. We are now at odds over this brat child. I really love him and he is great with my kids a very loving sweet man but is this something we can get over. He realizes all of our fights have been over his kid. He realizes she is a piece of crap but I do realize she is still his kid. How do go back to the once loving happy couple we were? This kid has a kid on the way how do we deal with that because she is not welcome here. The environment this kid is going back to is nothing but weed, drinking and parties how do I deal with knowing the grand child will be growing up in that environment? I have never dealt with a kid that is like this and my house has always even before my SO been a place for wayward kids but I have always helped them learn. His kid is impossible and worthless. I have never felt this strongly negative about a kid before. Not sure how to handle this. Just feeling lost and confused.