What is the ruling regarding my maternal aunt's husband and her children, and those whom she is currently nursing, after my maternal aunt nursed me for 20 days, given that she has one son from her first husband and five children from her second husband?

You become your aunt's daughter through suckling, so her children are your siblings, and your former uncle (aunt's husband) is a father to you and his children are your siblings, based on the Prophet's (peace be upon him) saying: "What is forbidden by lineage is forbidden by suckling."

Your current uncle (aunt's husband) is a mahram to you because you are his stepdaughter, in accordance with the verse: "{And your stepdaughters who are under your guardianship, born of your wives to whom you have gone in}."

As for his children, those who are from your aunt who suckled you are your siblings. Those who are from another woman are not your siblings, and you may marry them unless there is another impediment.