Check the buck for issues that might make him uncomfortable - sore hocks, sores on his genitals or inside his legs, etc. I've seen all of those things make bucks change their mind during breeding attempts. Recently we had to help my daughter's friend with a buck that had gotten "stuck" in his privates, if you catch my drift. When we examined him, we couldn't express his penis, in fact when we tried, it prolapsed (turned inside out). We soaked him in warm water for some time and carefully worked everything loose. He seems
much happier now but she hasn't tried breeding him again yet. He's an older proven buck and the only thing we could think was that he possibly got dehydrated...?
Keep track of your buck's weight - that can sometimes give you a hint that something's wrong. Make sure he's healthy and then I'd give him a break and let him try again later before culling him. Everybody has their off days.
I'd say breed your doe to whichever buck you like better, sire or sib. She is definitely 50% genetically related to her sire. On average, siblings are also related at the 50% level, but theoretically they can share 0% to 100% of their genes (of course the extremes are far less likely). Unless you're aware of issues in the line - malocclusion, tendencies to GI problems, etc. - I wouldn't worry too much about it.