Of course it is. What made you think it isn't?
So by that logic adopted children shouldn't have the rights of biological children when it comes to family issues, and brother-in-laws, mother-in-laws etc. are meaningless, not to mention the fact that a husband and wife are not genetically related.
Not everything about family is procreating and genetics numinus - just because we live in an age where the nuclear family is encouraged doesn't make it the only way family can be.
Marriage, in the same vein, is not all about creating children.