Yes, it's fine to breed the same bloodline for several generations without any ill effects.. we have a running experiment with a green/black auratus bloodline that we are constantly breeding siblings we are now into the 9th generation and have noticed no reduction in production or quality, though there is a slight discoloring after f6 or so.. So you are fine with your frogs breeding for quite some time.. Peter Keane, Curator of Reptiles, JungleWorld
The other posters are correct in their responses on this matter. Even past 12 generations it is still possible to breed siblings without "ill effects". I say "ill" because many things can remain latent. Some will look fine, but there could be a decrease in immune system function or other unseen problems. I personally choose not to cross siblings but will occasionally cross "cousins". Cousins average roughly 25% (but can be anywhere from 0-25%) of the same genes so I consider them somewhat unrelated.
Inbreeding basically causes a buildup of recessive traits over generations and the general thought is that recessive genes are often deleterious, which is not always true.
My advice to you is that you could cross the siblings and then trade the offspring for other individuals of the same species- then setup nonrelated pairs. Or an even better idea is to just trade one of the siblings and completely avoid this whole ordeal.