Depends on preference and purpose really. Bother to sister does happen from time to time by mistake, looking for an unrelated buck is always beneficial overall, but if your breeding for form or production you want to look for good bucks with good form or from good productions lines.
Like stated above if you are going for quality, showmanship and bettering the breed..buy the best and unrelated buck you can afford that will improve on your ladies. If udders are not great, find a buck whose genetics show beautiful udders. Im a firm believer you should always breed to better the herd, But if breeding for meat purpose, then we can loosen up a little.
I have bred 1/2 sibs together out of necessity, with two different groups..both Sires and Dams were of nice breeding blood. They say the difference between inbreeding and line breeding is the end result. One worked out wonderfully..kids are always fantastic (we call this line breeding)..the other we got a hermaphrodite (we called this Inbreeding) Of course we never repeated that breeding again..just did not work out. Many feel grandfather is the closest you should breed. I was also recently informed there is a calculator available on line that you can calculate how close genetically are Buck and do. I have not looked it up yet. Hope I didn't confuse matter more LOL
I read am article a few years back about the benefits of intensive inbreeding. The paper was written about dogs but I would assume that it would apply to all domestic animals. Like a mother to son and direct siblings back to each other for several consecutive generations. The premise was you'll so greatly magnify the negative traits that the genetic stock will go through a period of very high defects resulting in necessary culling (like 20-50% of the offspring if I remember the paper)
But then afterwards when new outside genetics is introduced all of the negative traits had been culled out and the remaining stock was superior. They basically forced all the bad stuff to boil to the surface.
Anyways a breeding system like that would lake years and I'm not sure how I or you feel about that much intentional culling, but it was an interesting perspective on linebreeding.
Yes I think very intense line breeding you almost have to start with absolute perfect animals to start with. There’s one breeder I follow on FB that has just absolutely stunning goats. I started looking at his lines and he line breeds like crazy. But he also has totally flawless animals and who knows what he needs to do not posted on FB ya know.
Personally though I have bred half brother to sister and sire to dam, there has been some in there where it’s like the great grand sire of one was the sire of another. Most of the time the cross was great, some not so great. I personally feel that I have a lot to improve on and I do not have absolutely 100% flawless bucks so I don’t go out of my way to line breed. I want to improve my herd not stay where I’m at. But when something comes up and I have no other option but to line breed or pair with a buck that I don’t think is going to produce the best kids, I line breed.