It remains to be seen whether "Grandpa" is merely a term of respect between generations or actual bloodline. If the latter case, then someone should tag as "incest."
I'm not certain of it either; "ojii-chan" can be taken as "grandfather", but it could also merely be an old man that she is very familiar with. The language is somewhat ambiguous in that respect.
Do my breasts taste good, Grandpa?Jerk jerk jerk Splurt