You shouldn't avoid your parent's significant other.
Dan calls his father Mark to let him know he is coming over, but learns that only Mark's new wife Sarah is at the house.
Dan wants to pick up a lawn mower Mark has decided to give to him.
Dan stops inside to say hi to Sarah and see how she is doing before inviting her to point him towards the lawn mower.
Dan enters the house through the garage and picks up the lawn mower without saying anything to Sarah, then leaves abruptly.
Sarah feels included as part of Dan and Mark's family, improving her relationship with Dan and his relationship with Mark.
Sarah will feel as if Dan's avoiding her, which will poison Dan's relationship with his father Mark.