I know people have mentioned it but just to hammer this into the ground - the first half of the Japanese title is 'ママの妹の童貞教育'. Which reads '[Mommy]['s] [Younger Sister] [virgin] [education]'.
The Japanese title could have just gone with '叔母' (aunt) but did not, going with 'ママの妹' instead. So in this case the translation as 'Mother's Sister' is more accurate, it's just as awkward as the original title. I would have gone with 'My Mother's Younger Sister', but that's just editing for readability and hawtness.