A blood relative; someone related by blood (as opposed to by marriage).
肉親が遠くに住んでいるので年に一度しか会えない。
My blood relatives live far away, so I can only see them once a year.
事故で肉親を亡くした人を支援する活動に参加した。
I took part in activities supporting people who lost blood relatives in the accident.
結婚すると法律上は肉親の範囲が変わることもある。
When you get married, the legal range of your blood relatives may change.