It will be fine because the mating session won't or should I say shouldn't result in little one's!.
I say shouldn't because the vasectomy isn't always a success.
But as long as he is definately firing blanks! it will be ok, as the end result won't be little ferrets with genetic problems.
Mating siblings doesn't always result in problems, however it's always best to keep a healthy blood line by mating unrelated healthy ferrets.
Interbreeding isn't good with any animal