Relationship messes which is ethical or unethical in your opinion?

In this messy, messy mine field of what people call love and relationships which scenario would you figure ethical or unethical?

Scenario ‘A’ Bill and Brenda had a long term relationship, but it ends badly, because of their relationship Bill got to know Brenda’s BFF Alice who remained friends and became an item. Is it ethical for Bill and Alice to get together seeing Brenda’s feelings and ego might get bruised, or is it unethical?

Scenario ‘B’ Bill and Brenda had a long term relationship, but it ends badly, because of their relationship Bill got to know Brenda’s younger sister Alice who remained friends and became an item. Is it ethical for Bill and Alice to get together seeing Brenda’s feelings and ego might get bruised, or is it unethical?

Scenario ‘C’ Lars finds his g/f Tina’s young mother (had Tina in her teens) hot, and when her mother’s marriage fails and she divorces he dumps Tina to move in on the mother, which takes the bait and they become an item. Is that ethical or unethical?

Scenario ‘D’ Betty has a BFF she brings home for the holidays from college, the BFF and her divorced dad hooks up and becomes an item, ethical or unethical?

These days with no holds barred, life is short, get yours now and be happy climate is there really anything unethical so long as no one is cheating and above the legal age?