Progress as far as you can with Mary. Even once you have had sex you might need to go back so kissing, or some of the other lower steps with her. In the meantime keep playing around with Alexa, try to sneak into the restroom, etc. I think either in the morning or the evening when everyone is home make sure you go up and down stairs a 2 or some times, it might trigger a little fight between them. Eventually this will up the ante that Alexa wants you to do stuff like take kinky selfies with her, eventually hand job, I think then BJ, then anal, then finally take her virginity. Once you have has full sex with both of them, just keep that up for a bit and every time they both try to spend the night with and start fighting just keep trying to make them kiss, then get naked and kiss, then jerk off over them while they kiss which will give you the option to have sex. Most of the time they leave the room but I think after you take both of their virginity you can get them to let you have a 3some with them. Shortly after that I was able to have a 4some with the step mom but she mostly just assists and does not join in.If I remember correctly (and it has been a VERY long time since I did this), with Mary you can visit her in her room and offer girlfriend training, you start with flirting, and then move on up, at some point you will max it out, at this point you need to return to flirting, and you can repeat the steps, only with fewer clothes. I can't remember if you have to do this a couple of times to get her to be your actual girlfriend.
I'm less sure about Alexa, you can try watching TV with her when the other go to bed, that will get you some Sexual tension, then you will have to do things (like make her cum in her room), I think there is a random element where she had a fight with Mary, and pretends to have a boyfriend, and she will get you to go into her room and pleasure her, this should get you on the right path I think, but I am super hazy on this.
Edit: Another thing you can do to increase sexual tension with Alexa is to jerk off solo in your room (in the second commune house), sometimes she will catch you in the act.