1- There is 3 type if time limitation, the first is, this event will happen at this day of the week at this hour every weeks, the second is, this event need a number of day to pass to progress or to trigger again, the third is this event will happend in a certain amount of days, should you miss it, you must trigger the event again.
2- Both sister will sometimes need you to do older scene to trigger new one, you also need to catch them when they argue in the stairs, this event seem to be hard to trigger, personaly, it happened to me when I was going upstaire after dinner.
3- I'm not sure where exactly you are in the relationship with her, but maybe, try to sleep with her, wake up at 8, try to peek at her when she is in the bathroom, if she catch you she will let you come in, do that again and ask her to take a shower, do that several time and you might end up fucking her.