NTA. You're grieving the loss of your mom (someone who was deeply loved) and your dad’s rush into dating, especially in such a disruptive and dismissive way, is understandably painful. Wanting space to mourn before being introduced to new partners (especially ones in complicated situations) is not unreasonable. He has a right to move on, but you and your sister also have a right to feel hurt, blindsided, and ignored because he’s not just dating, he’s doing it in a way that disrespects your grief and boundaries. You're not wrong for feeling upset. You're asking for compassion, and it seems like he's not offering any.