Summer, With Nieces

The ending is mostly decided on the first day + 2nd day's Morning.

First of all, going for either girl on the first period (1st day noon) pretty much have 0 effect toward ending (you can't have sex with the girl then), so frankly I recommends grabbing the aphrodisiac here.

For evening/night action, either have sex with both girls for ending 4/5/6/9, neither for 1, or go only for one of them (you only need to do it once, having sex twice doesn't add to it) for 2/3/7/8

For 2/3/4, you can stop doing anything from this point to the ending (you can do the 2nd day morning stuff but no more than that)

For second day morning, you either visit one of the girl (going for her route, i.e. ending 5/7 for Miyuki, 6/8 Hinata) or visit the kitchen if you had sex with both (which should now have a question mark on it) to use aphrodisiac.

At noon you go outside and this more or less lock you into the route.
(too lazy to try whether going for one girl here then go after the other at night will get you enough point for 3rd day harem route)

Alternate version for easier reading:

First day starts at Noon - Pick the Aphrodisiac (white/pink bottle, the other one is energy drink)

Evening - have sex with one girl unless you're doing ED 1

Night - have sex with the other girl for 4/5/6/9 or you can do nothing for 2/3/7/8.

2nd day Morning -
If you had sex with one girl - go for her if you want ED 7/8. Not doing this guarantees 2/3, but doing it doesn't prevent you from remaining in 2/3 until you do the Noon scene.
If you had sex with both - Go to kitchen (with ? mark) for ED 9, go for one girl for 5/6, do nothing for 4.

Noon - Don't go outside for 2/3, go outside for 7/8/9.

For single girl route, keep going for her just to be safe, for harem route it does not matter after that noon scene you're good (though whoever you go for at night will give you blowjob the next morning)

Hinata = MC's older sister's daughter. The sister divorced/broke up and returned home.

Miyuki = MC's older brother's daughter. Both husband and wives are alive, they inherited the house after the parents died long ago.

The reason you're left alone with the two girls is just that they're super busy so they left early.
And as for why the game have time limit of 3 days, that's how long the MC is staying at the house for (before returning back to his job)