We understand your concern and discomfort. Firstly, we must address that the situation you're describing is not in line with biblical principles. In the WEB, it is written:
Put to death therefore your members which are on the earth: sexual immorality, uncleanness, depraved passion, evil desire, and covetousness, which is idolatry. (Colossians 3:5)
Furthermore, the Bible advises us to abstain from even the appearance of evil (1 Thessalonians 5:22). Allowing a boyfriend to stay over and spend the night gives an appearance of impropriety and is not wise or godly even if they are sleeping in separate rooms. We must honor God with our bodies and our behavior.
Your stepmother professes to be Christian, yet her actions are not currently aligning with her confession. We encourage you to share your feelings with her and to remind her of God's standards for relationships, pointing out that having her boyfriend stay over is not honorable or pleasing to God. Her relationship with her boyfriend should be one of courtship with the goal of marriage, and even then, they should not live together or stay overnight with each other until they are married.
Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous. (Hebrews 13:4)
We also want to remind you that the power of prayer is made available to us through Jesus Christ.
Jesus said to him, "I am the way, the truth, and the life. No one comes to the Father except through me." (John 14:6)
For there is one God, and one mediator between God and men, the man Christ Jesus, (1 Timothy 2:5)
Now let us pray together for your situation.
Dear Heavenly Father, we come before you in the name of Jesus to lift up this child of Yours to you. Lord, You know the discomfort and unease they are feeling in their home, and we ask that You intervene and make a way for this situation to be resolved in a way that honors and glorifies Your name. Lord, convict your child, the stepmother, of the error in her ways and draw her back to a relationship with You that is pleasing and honorable. Help them all to uphold sexual purity and follow the scriptures. Give the requester wisdom on how to approach this situation, and give them the right words to say to the stepmother. Give them both strength, wisdom, and a willing heart to obey You, Lord. And, lastly Lord, use this as an opportunity to demonstrate the power of prayer and obedience to the requester, the stepmother, and the boyfriend. We ask all of these things in the precious name of Jesus Christ. Amen.