When Sakuya refers to "Imouto-sama" she probably talks about Flandre. (Just for context)
Also it actually reads "Masturbating to pics of big sis Sakuya" or "Masturbating to pics of Sakuya onee-sama.
The first one is absolutely correct, the second one, it's up for debate. If you check the comments in Pixiv, people are wondering whether there's an implicit comma or not (as in 「さくや姉様の写真でオナ」—Masturbating to pictures of Sakuya-nee-sama— or 「さくや、姉様の写真でオナ」 —Sakuya masturbates to pictures of nee-sama). Given the subject at hand and how the meme usually goes, I would imagine the latter would be the most likely option.
This is definitely about the "Goddess Fall" incident.
For those that don't know:
Nikke were revered as the saviors of humanity, but after a catastrophic failure in a pivotal part of a war, the human's perception of them changed from Heroes to Expendable tools. They were treated as 3rd class citizens (at best), and any human advocating for their rights were labeled as terrorists and subjected to persecution 200 Mass-produced Nikke decided to leave and build a home for themselves. As they boarded an elevator to the surface, the government cut the power to stop them. The elevator was 4km high when the power was cut and was now on a collision course. The Nikke decided to save the humans below by sacrificing themselves by self-destructing and blowing up the elevator.
This is the reason why Nikke squads are 3 or less...
This is definitely about the "Goddess Fall" incident.
For those that don't know:
Nikke were revered as the saviors of humanity, but after a catastrophic failure in a pivotal part of a war, the human's perception of them changed from Heroes to Expendable tools. They were treated as 3rd class citizens (at best), and any human advocating for their rights were labeled as terrorists and subjected to persecution 200 Mass-produced Nikke decided to leave and build a home for themselves. As they boarded an elevator to the surface, the government cut the power to stop them. The elevator was 4km high when the power was cut and was now on a collision course. The Nikke decided to save the humans below by sacrificing themselves by self-destructing and blowing up the elevator.
This is the reason why Nikke squads are 3 or less...
The artist posted this on Christmas day. In Japan even more than in Western countries, Christmas is thought of as a couples' holiday. They also have the tradition of eating chicken for Christmas dinner. The implication here is that this girl is buying her Christmas dinner from a convenience store and she's embarrassed that the shopkeeper will be able to tell that she's single because she's only buying enough food for a single person.
You could read into the situation more, but that's the basic gist.