related by blood
"Although they grew up together, it turns out the two men are not merely friends but akin as first cousins."
Of the same kin; related by blood.
"After discovering they share the rare blood type, the two strangers realized their families were actually akin and decided to meet for coffee."
A surname.
"The local historian noted that the estate's original owner, Mr. Kin, was actually spelled akin in some early records due to a clerical error."