Hi
I have in the past had a bitch come to stud that was a half sister. The 2 litters she had (18mths apart) were lovely. I have advised however that if they plan on mating a bitch from this combination then they need to completely outcross for fresh blood, then come back inline again.
I am not afraid to do a grandfather/grandaughter mating either, which when talking to some other breeders say this is a classic mating.
Mother to son, father to daughter i think personally is too close & the KC have now disallowed these kind of matings.
Fresh blood is a necessity in a breeding programme & i think that it is good to outcross completely sometimes, although to keep a 'type' then line breeding is also a big part.
My puppy 'Bunny' is line bred to a point, but also has been outc rossed. She is from a mating of 'cousins'. Pumba is a son of Giorgio x Angel. Queenie is a Daughter of Danny x Amber. The tie in here is Danny is Angels Brother. However, Angel is not only Bunnys Grandmother, she is also her Great Aunt. Danny is not only her Grandfather, he is also her Great Uncle. The total outcross in this breeding is Giorgio, he is completely american bred & has no ties at all to either pedigree.