I'll add to the Grandson path, as best I can. I use the Cheat QSP so things maybe slightly out of whack.
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When Yana is of high enough stats, talk to her in the kitchen at 9am. Ask about Julia. Dunno the exact required stats, I just maxed them all.
Go to Julias house in the suburbs sometime in the afternoon and greet her as her Grandson. She'll leave and the MC will be like "Maybe I shouldn't have done that".
(Maybe only on weekends) Go back to Yanas room late in the day and there should be an option to talk to her again about Julia and the money problems. I didn't read the exact words on the option, mostly because I accidentally clicked it, and barely registered it was there at all. Should be obvious tho. I missed this option for quite a while, since I never talked to Yana again after getting Julia open, and was wondering why Julia never showed up.
After you do the above, Julia will randomly appear in your house. If there's a time requirement for this, I don't know. She appeared for me at 20:00 on a Monday. I'm gonna say she just appears whenever you go back home once the requirements are met.
She asks for money, give her 100,000. If you don't have 100,000 you probably fail this and the path may close. Not sure, didn't try. This puts you on the full Grandson path, and you can re-enter Julias house at your leisure.
You can Talk to Julia in her Kitchen @ noon to go shopping(random), Sunbathe at the pool, and sleep in second bedroom. She'll randomly ask for money at various times. She only asks a couple of time throughout the week, resets every monday. You can also find her in the club on the weekends.
This is all padding. You're goal here is get about 300k total given to her. I tried smaller amounts, but it didn't work far as I can tell. If you don't use the cheat QSP, this will be quite a grind since she only asks for small amounts (10k at most) a few times a week(3-5).
Once you've given the required amount, leave her house and return. She calls you out for being a pervy peeper but says it's fine because you've literally made her rich. You can now do more things with her. Including, no big shocker here, give her lots of money.
You can take her on dates @ 18:00 from her bedroom(weekends). At the pool you can give her a massage and 10k and various other minor events are changed.
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At this point I got bored and vaguely lost. I'm 90% sure you need to start denying her money after a while. Eventually a talking point comes up in her Hall while watching TV that switches the relationship to full sexy times. I have a save before I cheated it, and may try it again later legitmately, but for now I just cheated the "Stood up" variable to 5 and the relationship went to sex.
Doing this without cheating, the Cheat QSP variable tracker never went up when I denied her. So there's either a random timer here, the denial only goes up on certain events, a convo elsewhere I missed, an event I missed, or it's a massive very expensive grind. However my guide thing above should get you pretty far into it.