Follow the instructions of the Family Union quest in the OK Notes under the character button.
The general gist of it:
- Get both of their affection rate to maximum - 45.
- Talk to the step-dad to begin the throuple story.
- At this point, I recommend you do the Shifty Hooker quest to change the step-brother's shifts to the morning because all of the following steps require they are at home at the same time and you can catch them there in the evening.
- At first, have sex (regular handjob, blowjob, fuck) with either of them to reach 5 acceptance.
- Then, talk to the step-brother to begin the threesome scenes.
- Right off the bat, there won't be many threesome interactions because they're still not used to with having sex with you in the presence of each other.
- However, once you reach higher acceptance, you will unlock more choices during the scenes including the threesome options.
Hope this helps!