Ichipapa is 13 years older than Ichimama, however Ichimama is 46, while Ichisister is around college age. Ichisister's age should be around 18-22, so if we took the worst case scenario of Ichisis on her last year of college (assuming she hasn't skipped any grades), at 22 years old, Ichimama had Ichisis at age 24. At that time, Ichipapa would have been 37. If we apply the formula of half your age + 7... at 37 years old, Ichipapa's minimum allowed age should have been 25.5... And that's only considering the circumstances of Ichisister's birth. If they had been dating prior and had gotten married before doing the deed, then the disparity of Ichipapa's minimum allowed age and Ichimama's age should have been even wider when they begun the relationship.
Ergo, Ichipapa is indeed a cradle robber*.
*Of course, this is assuming that it was Ichipapa who initiated the relationship. If Ichipapa is anything like his son, then it might have been possible that, like Ichikawa, Ichipapa was a loner who gave off vibes that activated the maternal instincts of Ichimama. Thus, the possibility that Ichimama was the true criminal (the crime being that of grave robbing) is indeed not impossible.