If you have two siblings from a set of well mixed parents, the odds are likely lower because it is likely that only one parent had the gene at issue.

However, the taboo of marrying siblings and first cousins are usually cultural, which means the likelihood of both parents having the same gene which will cause a genetic issue. So, if you have a human population with a smaller overall gene pool and/or families where intermarriage within a family is high, the odds of genetic disease from familial interbreeding spikes.