There are families that for whatever reason are unkind to new people. It may be a culture of suspicion or jealousy. Its also that its possible that the family considers itself a closed system and “outsiders” are not welcome.
Most people want to introduce a new important person to their family. It cannot be shocking and embarrassing when a parent or sibling behaves rudely or ignores the introduction. The first course of action is to apologize to your sweetheart. Do not act like the behavior is acceptable or not happening. The next step is to leave. The new person in your life should subjected to your family’s rudeness.
When you are alone with your family members, ask them gently what was behind their behavior? Explain that you are hurt by it and want to understand. If it continues, tell your family that you will no longer bring new people in your life around for them to meet.
If possible, family counseling would be very beneficial to help you and your family understand the unspoken dynamic around “outsiders”.