Relatives; one's family (people related by blood or close family members).
身内に医者がいるので、手術の相談がしやすかった。
Since I have a doctor in the family, it was easy to consult about the surgery.
葬式は身内だけで執り行った。
The funeral was held with family only.
身内の不幸で、週末は帰郷することになった。
Because of a family bereavement, I have to return home this weekend.
Friends or members of the same group/organization — people within one's inner circle or close associates.
会社の重要な仕事は身内だけで回していると批判された。
It was criticized that important company work was being handled only by insiders.
政治家は身内を優遇したと報じられた。
It was reported that the politician favored his inner circle.
イベントは身内のメンバーだけで準備した。
The event was prepared only by members of the inner circle.
Followers or henchmen — used for close aides or entourage, sometimes with negative connotation.
あの経営者は身内を大勢引き連れているが、批判も多い。
That executive brings a large entourage of close associates, but he faces much criticism.
逮捕されたのは政治家本人ではなく、その身内だった。
It wasn't the politician himself who was arrested, but his associates/henchmen.
An older/less common usage meaning one's whole body or one's internal affairs.
古い文献では『身内を清める』といった表現が見られる。
In old documents one can find expressions like 'purify one's body/internal affairs.'