So, from what I can tell, the aunt and nephew had a sexual relationship 15 years prior to the current plot, while the nephew was still in high school (pages 2 & 3.) She fled to Tokyo because she felt guilty for having a sexual relationship with her nephew, and then proceeded to get married to her current husband and have two children. She told neither her nephew nor his mother (her older sister) where she went. Fast forward, 15 years later, they happen to start working at the same company, and go out to a mixer with other co-workers for drinks/reunion. She gets hammered, and the nephew and coworkers brings her to his place where the nephew proceeds to make his move. Of course, their having had a physical relationship before doesn't make the seduction all that difficult. And one more thing, a common theme here is that she's in her 50's.
@NewtBlack Some say that cum of young boi(aka Doutei in japanese) has magical effect of antiaging...with unholy power of incest, even better of course. Must be true, I studied this theory with thousands of reference in the ocean of manga.