Inbreeding is said to be safe for 6 generations (siblings). But I wouldn't go that far. I prefer to breed cousins, daughter to father, son to mother, then 2nd cousins and uncles etc. I would only breed siblings of the same batch if I had no choice and after F4, I introduce new genes/fish.
Inbreeding is done if you have certain recessive traits that you want to make dominant. Eg. HM single tail with long/large dorsals. But the side effect is that you tend to make defective genes dominant as well, specially if done for too many generations. It is highly advised that you don't inbreed too much - unless you are trying to make certain traits.
Aquabid usually offers "pairs" of siblings. You need to ask for a non sibling if you don't want to inbreed. But you can breed siblings for 4 generations and theoretically you should have offspring with the same traits (color/patern/form) as F0 (the original parents).