I would say based on the definition of moral it is not morally permissible.
1)Based on personal conscience.
2)according to common standard of justice
3)encouraging goodness and decency
4)good by accepted standards
5)able to tell right from wrong
6)based on personal conviction
.
I don't believe there is such a thing as interracial relations but I only see 1 and 6 as possibly being against
2) Standard justice accepts interracial relations in most states and countries does not accept siblings
3) interracial relations would strenghten the species as a whole. siblings relations would thin the blood should an accident occur.
4) interracial relations are good (interreligious may not qualify here). Sibling relations not good.
5) able to tell right from wrong No issue with interracial major issue with sibling.
Only negatives are 1)based on personal conscience or 6)conviction or basically being racist.