two daughters "rape" their dad with help from mom, the become pregnant, it is discovered and dad is seperated from wife and daughters.
dad has a hard and lonely life untill he moves into an Apartment complex, where a women lets him have fun with her Little girls.
ends with dad and wife + daughters being reunited.
That's The Pedophile, by Frank McCoy. You won't find it on SOL as it violates the site rules (sex with under 14). It's probably still kicking around ASSTR or one of the sub-sites.
Be aware that some or all of Frank McCoy's stories are currently considered to be "obscene" under US law. The author was convicted of "transporting obscene material across state jurisdictions" in 2013 in a district court in Georgia, and his appeal was denied in 2015.
