OK, I managed to break the game somehow.
1-Keep relationship with mom + while with sister -
2-Keep them balanced, like +20 -20 and so on
3-Sister bedroom handjob event with Devon triggered
4-Mom handjob event with MC triggered
5-Devon event triggered without mentioning sister
6-Refused the bastard with -20 relationship
7-Focused on sister till Devon and her had sex
8-Focused on mother and had sex with her
9-Devon got kicked out
10-And still sister events are triggered with Devon after he got kicked out
11-Set the relationship with sister from -60 to 0 with cheats (Otherwise I had to repeat Sister's Room 16.00 event at least 60 times, screw this)
12-Sister events triggered properly but Devon sister bedroom handjob event triggered until MC sister shower handjob
13-After that event triggered I had no issues with MC sister bedroom handjob event at 1200
14-Rest of the events triggered without problems, including the yacht sex event
So, in conclusion, having both female characters, letting Devon have one of them and kicking him out later is possible in about 70 days in game (130 without cheating at (11))
I might test later whether Devon having them both and kicking him later is possible. (Imagine mother getting pregnant from both Devon and MC wtf lol)
So many things have to be fixed.
P.S. Also checked that route and it seems that pregnancy locks the path for mother. You can seduce back the sister but not the mother if pregnant.