If TV characters are about the same age as the actors who play them (unless specifically mentioned otherwise), then there was nearly a thirty year difference between Blake Carrington and his younger brother Ben (played by John Forsythe and Christopher Cazenove, respectively).
And it's not a case of Blake's father remarrying or having a son by a mistress. In the show 'Dynasty', Blake blamed Ben for the death of their mother. (Blake was supposed to be caring for her at the time.)
If the age span does mirror the actors' ages, then Mother Carrington gave birth late in her reproductive years. At best, if she married at 16, let's say, then she was nearly 46 at the time.
If Blake was so concerned about Mama, he should have had her moved to Denver where he could have kept an eye on her in her last years......