The description and game text says "Senpai" (Senior colleague) but it's strongly implied that Shizuka is actually Sewa's Aunt, a blood relationship.
I've played this game through, they aren't related. If the term "aunt' or "obaa-san" came up anywhere it's a loose term that can also be used for any older lady related or not. The terms, "senpai and kouhai" are explicitly used designating the two woman's relationship.
I was about to say "What do you mean 'Implied'? Do they not tell the kid this woman is his aunt?"
The DLsite page doesn't have the Incest tag either, and the title explicitly being "Mama's Kouhai" with Senpai and Kouhai not being used to address family. At least not normally.
And as TigerC2 said, if these implications you meant are something like the terms "Aunty" and "Obaa-san" then it isn't necessarily imply a blood relation. They are simply what you would use to address an older woman of around middle age as a younger person.
If you could, please describe the strong implication you found. It would really help.
Of course, as I said before the translation may not be entirely on point (shocker) especially when it come to addressing someone in Japanese, and that they tend to be become too literal when machine translated.