breeding bro and sis

is it ok to breed a male and female from the same egg sack

yes

There is at least anecdotal evidence that inbreeding will produce the same affects as in more familiar animals like dogs and cats etc .

e.g. stunted grown , problems shedding and reduced fertility .

But the jury is still out so the final decision is yours .

There's quite a few responses to this in various forums and the threads usually end up with plenty of strong views, good healthy debate IMO.
I know breeders that have no problem breeding incestuously and have done so through generations without any obvious ill-effects. Many people say that inverts do not suffer the same issues as mammals and reptiles and where small colonies arise it seems very probable that the gene pool could be limited.

However, as Mutley suggests there are people who believe the issues will be seen in future generations and only time will tell.
To answer your question correctly, its not very likely that you can breed most species with brother and sister as nature has its own protection here. Generally males will mature much quicker than females and therefore are ready to mate before the females from the same sac. The female may mature 1 or 2 years after the male at which point the male is likely to be past his sell by date.

I say generally because there are instances of males lasting much longer and still mating, I can think of a L. parahybana that is past 2 years, if I'm right up to 4 years old, and still mating! Also its possible to speed the maturity up of females and slow the males down by adjusting the heating and feeding. What was recently relayed to me when I had supposedly 2 males and 1 female P. rufilata, was that the males presence (even in the same room) could trigger the female to repeatedly moult to achieve maturity. This often results in a shortened lifespan for the female and may even result in her death before maturing. (pers. com. R. Gabriel)