🏛️ Mormon / LDS
They say

“Jesus and Lucifer are spirit brothers—both spirit children of Heavenly Father and Heavenly Mother, with Jesus as the firstborn.”

The 30-second answer

Jesus is the uncreated Creator of all angels, including Lucifer. Satan is a created being, while Jesus is the eternal God.

💡 Ask them back“In Colossians 1:16, the Bible states that Jesus created ALL things in heaven and on earth, visible and invisible, including thrones, powers, and rulers. Was Lucifer created by Jesus, or did Jesus not create all things?”
⚠️ What not to sayDon't use this as a shock "gotcha," and don't let them redefine the word "brother" to avoid the doctrine. Keep the focus on who Jesus is.

📖 The full defense

Jesus is the Creator of Everything:
Colossians 1:16-17: "For by Him all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him. And He is before all things, and in Him all things consist."
John 1:3: "All things were made through Him, and without Him nothing was made that was made."
If Lucifer was made, Jesus made him!

Lucifer is a Fallen Cherub, Not a God:
Ezekiel 28:13-15 reveals that Satan was "an anointed guardian cherub... blameless in your ways from the day you were created, till iniquity was found in you."

Jesus Has No Beginning:
Micah 5:2 reveals that the Messiah's goings forth "are from of old, from everlasting." He is not a created brother; He is God Almighty.

✝️ Scripture anchors (NKJV)

Colossians 1:16-17
“For by Him all things were created that are in heaven and that are on earth, visible and invisible... All things were created through Him and for Him.”
John 1:3
“All things were made through Him, and without Him nothing was made that was made.”

🔁 If they push back…

The comeback you are most likely to hear next — and a reply that keeps the conversation moving toward Jesus.

They say“Colossians 1:15 calls Jesus "the firstborn over all creation." So he was the first spirit child born.”
You can say"Firstborn" (prōtotokos) is a title of rank and inheritance, not necessarily birth order. God calls Ephraim His firstborn though Manasseh was born first (Jeremiah 31:9), and promises to make David His "firstborn, the highest of the kings of the earth" (Psalm 89:27). The very next verse says all things were created by Him—so He can't be one of the created things.
They say“All of us—including Satan—are Heavenly Father's spirit children, so calling them "brothers" just means we're all family. You're making it sound worse than it is.”
You can sayI'm not trying to make it sound shocking; my concern is what it says about Jesus. If Jesus is one spirit child among billions, He is a created being, however exalted. The Bible says He is the uncreated Creator who "was in the beginning with God" and "was God" (John 1:1-3). Only God can bear the sins of the whole world.
🕊️ Pentecostal key — the Spirit on the streetSpeak with compassion to LDS missionaries. Many are far from home and under immense psychological pressure. Offer them a cold drink, hospitality, and pure Christlike love.