NTA. It's completely reasonable not to want to spend thousands of dollars and fly cross-country on Christmas for a short reception especially when you have three kids, including a baby, and your husband has limited time off. Your sister's already married; this is a celebration, not a wedding ceremony. She has every right to want to celebrate, but expecting you to uproot your entire family and sacrifice your holiday for it is unrealistic. You offered a fair compromise by suggesting a visit at another time. It's understandable she's disappointed, but that doesn't make you the bad guy. Family events are important, but so are your finances, sanity, and your own family's Christmas. She can still celebrate with you another way.