- They are related to each other being the descendants of James.
- They are related to each other as being the descendants of James.
In sentence no.1 "being descendant of James" is a participial phrase adding relation to two individuals, but the sentence sounds awkward to me, whereas the sentence no.2 sounds correct to my ears but I feel there is unnecessary repetition of "as" and "being"(we can use one of them).
I am no native speaker so, I could be wrong. The first one was used on family tree table or pedigree table which is a legal document.