It's not ideal even when it comes to line breeding but it has been done plenty of times. Half Sisters should be fine. Full sisters will most likely be fine but your working with less genetic diversity so issues would be more likely to arise in that pairing than if it was an unrelated pair or parent/offspring pairing.
I don't know if they are half or full as I bought them from someone with two clutches. But I have completely unrelated birds to outcross them with. I just wanted to do it one generation.
I know the Opaline Lovebird was bred this way as it was a rare sex linked mutation. They bred siblings and then outcrossed them and then bred them back to cousins to make sure they had the mutation.
I only planned on linebreeding for one generation to get the barred lilacs. I have been told that the more the parents were related the worse it is to cross siblings. This was by an Ameraucana judge. But if it is only done occasionally that no ill effects should come of it.
I just want to hear many opinions before doing it, if I do. I don't take having babies with issues lightly. I have had to put mutated babies down before that I hatched out of eggs I bought. I would rather avoid having to do it.