They are hard to get. For Alexis, you have to do a lot with Michelle. The 4chan thread has different info on it, but from what I can tell, you need to creampie Michelle like a hundred times, talk to her and she'll talk about a bunny maid outfit. Have sex with her in the bunny maid outfit a bunch of times and you'll get dialogue about playing with her anus. Eventually she'll get into anal sex, so you do that about five times. Finally, she'll talk about being unhappy about getting pregnant with someone else, then she'll start calling you master.
Afterwards, you work as a lifeguard and hopefully get a scene with Michelle and Alexis. Go and talk to Michelle asking about Alexis, and order her to set up Alexis for sex.
As for Miyu, someone said on 4 chan you need like 400 pregnancies, which is insane. I haven't gotten there...