It's polite to offer food to a relative's significant other.
Kathy brought a new recipe she made for the family's Thanksgiving potluck, where her brother Tom showed up with his new girlfriend.
Kathy wants to share her dish with the family.
Kathy offers her dish to Tom's girlfriend and makes small talk with her while they try everyone's dishes.
Kathy offers her dish to everyone but Tom's girlfriend and doesn't interact with her beyond introductions.
Tom's girlfriend opens up to Kathy, and they start to become friends during the meal.
Tom's girlfriend does not feel welcome at the Thanksgiving dinner and rethinks her relationship with Tom.