I am a father who for 20 years trusted in and raised 2 children. My oldest stayed faithful to the Lord as you are. My youngest has turned to the same place your brother has returned from.
I can only speak as a father here, but work “real hard” to find peace in your brother’s homecoming. It was because of the Lord’s love for you and your family that He restored your brother. It does sound like your brother needs to still learn lessons. Pray for him because if this is true than the dealings in all of your lives may continue for a little longer.
Stay close to His Spirit and He will show you both peace and rest in this. Family can cause use the most pain, but also the most growth. Jesus is closer to you in this trial than you know. Keep your faith. Pray that your heart not become harden because of this trial.
As a father I pray that God will do the miracle in my life that He has done in yours.