Imouto Haramasex

I only have my sister left in the family.
On her birthday, an angel named Coel appeared
and she announced that my sister is an incarnation of an angel.
She insists on taking her to the heaven, but I can never accept that because it means that my sister would be dead.
A mysterious priest then appears and tells us a way to avoid it.
"Your sister need to carry a new life in her body. Then they can't take her away."
Within the 15 days before the given departure date, can I achive results from our forbidden relation?

[From dlsite]