Incest is forbidden in most countries because if there is a genetic disease in the family the child of a incestuous relationship will be at least holder of the gene of that disease. But lets say the members of a family dont have a genetic disease and are not holders of any gene related to a genetic disease then theoretically would a child of a incestuous relationship not suffer from diseases by birth?
Incest doesn't necessarily mean that the partners are having unprotected sexual intercourse. It simply means sexual intercourse between closely related member of a family. In case of protected sexual intercourse, it doesn't matter whether or not the family has history of some genetic disorder.
But let's say they are practicing unprotected sexual intercourse. In that case, it does depend on the genetic history of the family. If either or both of the parthers has gene that result in a disorder, then thier offspring who gets that gene expressed in his/her body will suffer from that genetic disorder. And if it is not expressed, they become a carrier for that gene.
Now, coming to the case you mentioned, Yes, if neither of the partner have gene that causes certain genetic disorder, then the offspring will not suffer from the said gentic disorder.
But it only works if there's a no history of incest wihtin that family. In case they do have history of incest within thier family, they are likely to suffer from inbreeding depression. Inbreeding depression 'exposes' deleterious recessive genes by making them homozygous. This causes drop in fitness of the offspring and reduced fertility. Increased homozygosity is what causes issue, which increases chances of a harmful recessive gene getting expressed.
So it's not a big worry, if they don't have any such gene that cause obvious harm by resulting in a certain genetic disorder. But continuous incest in family may result in problems later on.
Anyway, it is possible to be able to determine compatibility of two people by checking thier genetic information [we may become even more advance in near future]. We can selectively edit a gene to make it so that it won't be present in the offspring. Gene editing at advance level will get really popular.