This is quite subtle fan service.
Any art image you find from Megami Magazine or Nyantype Magazine, be sure to add them there, too. Sonata, I added some of pics such as Queen Blade to those galleries.
This is from Nyantype #41.
Big Sister Maid is the older sister who is a former slave. Maoyuu's head maid take her and her young sister to be the new maids.
Submissions can take several hours to be approved.