Can we guess why?
The TV broadcast schedule of Recently, My Sister is Unusual on TOKYO MX and Sun TV was changed from the current 10:30pm slot on Saturdays to later at night, most likely in response to the initiation of debate by the Broadcasting Ethics & Program Improvement Organization (BPO) in Japan due to concerns submitted by viewers in Japan regarding the heavy sexual content present in the show. See the announcements posted by the BPO and some Japanese viewer reactions below.
Here is the news post on the BPO page.
"Recently, My Sister is Unusual on Tokyo MX and Sun TV debate initiated
Youth Committee at BPO has decided at the 153rd meeting on January 28th that the above mentioned program is to be one of the titles discussed for evaluation.
We have received several concerns in regards to the Recently, My Sister is Unusual anime that "the program included many sexual materials including masturbation by a female high school studentin the time slot that many teenagers could be viewing programming". After viewing the program, the committee decided to initiate an evaluation. Said anime program was produced by the production committee, but nominated Tokyo MX and Sun TV who programmed said show to be broadcast at 10:30pm for evaluation."
Then this news gets posted on the anime's official site.
via Recently, My Sister is Unusual anime official site
"[Tokyo MX / Sun TV] Broadcast schedule change starting with Episode 5
The broadcast schedule will change staring from episode 5 on Tokyo MX and Sun TV as below.
Tokyo MX Saturdays 22:30 will change to Tuesdays 25:30
Sun TV Saturdays 22:30 will change to Saturdays 26:00
*Episode 5 will air 2/1 on Sun TV and 2/4 on Tokyo MX"
Some Japanese reaction:
"So the 10pm time slot didn't work after all"
"Staff knew this was coming and this is absolutely appropriate. Should have aired after midnight from the beginning anyway"
"I have to change my timer recording. I'm not saying it's too much work"
"It left a name in the history of anime"
"The program included many sexual materials including masturbation by a female high school student in the time slot that many teenagers could be viewing"
"I wonder why the fact that voice actress Ogura is in the cast and a current high school student is not considered a problem"
"Recently, our TV censorship is unusual"