Family relationship between Mary and Elizabeth

When Angel Gabriel spoke to Mary about Christ birth, he also spoke about her relative Elizabeth - And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God. And behold, your relative Elizabeth in her old age has also conceived a son, and this is the sixth month with her who was called barren - Luke 1:35-36

And Bible confirms that Elizabeth was a daughter of Aaron from the tribe of Levi - In the days of Herod, king of Judea, there was a priest named Zechariah, of the division of Abijah. And he had a wife from the daughters of Aaron, and her name was Elizabeth - Luke 1:5

Then, How Elizabeth from the tribe of Levi can be relative to Mary from the tribe of Judah?

Actually Aaron's wife Elisheba from the tribe of Judah, So "Aaron and Elisheba" was great great "Uncle and Aunt" to Mary - Aaron took as his wife Elisheba, the daughter of Amminadab and the sister of Nahshon, and she bore him Nadab, Abihu, Eleazar, and Ithamar - Exodus 6:23

So there was a reason why Angel Gabriel mention Elizabeth as Mary's relative, and it's hard for us to remember all the past and family relationship, but God knows everything.