Please take a look at the OBJECTIVES tab and click on Family Union quest. It will contain the next step you need to do to proceed with the relationship.
The general gist of it:
- Get both of their affection rate to maximum - 45 and lust - 50.
- 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.
- During the threesome scenes, you only have 5 actions, you need to get both off in the scene for the acceptance to increase.
- 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.