What is even happening in this series. I can't even put names to faces. I'm so confused.

Base +6, floraider +6, Palaxius +33, EarthQuake +7, PetersPark +8, HCLI +6, slimyD +11, finnick8 +7, emmypacer +4

Well then from now on I will just come here for my dick.

Base +6

So the little sister got into some sort of accident and lost the ability to walk and have kids so she started a time loop, in the previous chapter we see her completely healthy and energetic because that's what she wants.
The guy has the ability to go back in time by fucking girls that has committed some sort of crime(more specifically, a regret for doing something), he goes back to before the crime(cause of regret) is perpetrated, if he fails to stop the crime(regret) from happening he gets into a time loop until he changes the future.
Up until now he was under the little sister's time loop effect so he doesn't remember that she was in an accident that killed their mother and paralyzed her from the waist down, he slowly makes his way back to that point so he can prevent the accident.

tl;dr
The guy is fucking his way back in time.

Base +7, Zetsubo Scynthe +7, blackkill +6, EarthQuake +7, Acerola +12, Kitawa +7, GearZeroSky +17, Und3adgam3r +7, IaY +8, drustrevi +10, Alaephant +2, Jago Smith +8, DCH268 +6, and 34 more...

Not just fucks his way back in time, he's fucking his way back to create his 3some of possible step-sisters.

Base +8, fishzz +4, finnick8 +7, karavanrob +13

The guy who fucked through time?

Base +23, kurzwaltz messer +6, Slyuse +5, azurecrymore +5, kasshoku +5, ReijiHHH +6, Aelth +6, Nicosai +10, davale007 +6, fishzz +4, finnick8 +7, iamaneskimot +6, sy8289 +10, and 4 more...

Building on that explanation, both his friend's older sister and his own older sister became aware of the situation through the repetition, ala Endless Eight. After managing to fuck himself from the 14th to the 13th, the friend's sister tried to kill him in order to create an infinite 14th day loop where he couldn't try and stop it (because she wanted to be in a Groundhog Day situation). He survived, and was stuck in the 14th day bedridden and unable to sex himself back in time.

His sister, becoming aware in this chapters loop, discovered that he was able to travel back to the 13th occasionally because he came in those girls more than once. Using this, she had her brother blow his load in her several times in order to send him back to an earlier day, aka closer to his younger sister's starting of the loop. He landed back on a day before his friend's sister became aware of the situation, and proceeded to use her as a jumpgate to travel closer to the point of causality.

Hence, we will dub this point he is trying to reach, where the time loop starts, the Crimes;Gate.

Base +6, azurecrymore +5, PetersPark +8, koihei +8, mendickantBias +6, Nelath +6, Aelth +6, BlueYaksha +6, Razeroc +6, slimyD +11, MiD0Ri +4, nihkilare3 +6, aurst +6, and 10 more...

Yuno? Haru? 13th?

This is like the Future Diary manga / anime.

Base +6

Fucking through time is so weird but still good

Base +6, Aelth +6, finnick8 +7, karavanrob +13

So what was the Harus sisters crime? from the way she stated it, it seems she did something that caused the accident that killed his mother.

Base +6

Its not "crime" per se. Its more "Guilt" or "Regret".

Because no matter how used to doing something "bad" you are, your conscience still consider it wrong, thus the "Regret".

As for Nee-chan, its that she regretted not taking Yuno home herself on the day of the accident instead of their mother.

Base +6

Bang 'em all to save everyone.

Base +6