
Personally I'd not repeat any mating that had produced retained testicles. Clearly if both are retained, that dog can't reproduce but if he has one, he could. As for breeding a sister, I'd not not, but I think I'd choose a stud dog that's not immediately related to the breeding that produced this fault.
As for delayed heats - sometimes this happens. And I tend to look at when mum/sisters/half sisters first started. Most of mine started at around 8 months and then cycled at that interval. Most - I had one who came in bang on 6 months, and then started cycling every 5 months ..... as she wasn't up to my standards, much as we kept her, I had her spayed without issue. She was a singleton puppy and I don't know whether her doing this had anything to do with that!!! My Whippet on the other hand, hadn't come in by 11 months at which point, despite him not wanting to spay her at 6 months, preferring her to have one season first, I took her back and asked him to go ahead. Her mum and half-sisters all came in between 12 and 18 months the first time. Could be this is a Whippet thing?
I don't know of anything to suggest retained testicles has anything to do with delayed first seasons.