家族 and 親戚: family and relatives
Separate the scope of a relationship from who attended a gathering.
Talking about family
家族 (かぞく) means family. 「家族と暮らしています」 means living with one's family.
「家族は別の町に住んでいます」 can describe family living in another town. Living together is not the sole criterion for being family.
Relatives
親戚 (しんせき) means relatives. 「親戚の家に泊まりました」 says the speaker stayed at a relative's home.
「親戚が集まりました」 says relatives gathered. It takes additional wording such as 全員 to establish that every relative came.
The scope of a gathering
「家族だけで旅行しました」 means traveling with family only. だけ restricts the participants.
「家族や親戚を招きました」 says family and relatives were among those invited. や gives examples and does not exclude other invitees.
Does 「親戚が集まりました」 mean every relative attended?
Show the answer and why
Not necessarily. It reports relatives gathering, but whether everyone attended needs separate confirmation.