Help, how to? (Incest in my family)

I come from a big family. I have three sister and one brother. So this basically what happened between sister 2 and my brother family.

My sister 2 has three sons and a daughter. My brother has three daughters. My sister first son (who I call call Sam) is in a relationship with my brother second daughter ( who I'll call Barb).

Sam is about 25 years old and Barb is about 19. They been together for about 5 years.

I personally keep away from them, and my brother and sister. I have always kept a closer relationship with sister 1 and sister 3.

My dilemma is I have a 4 year old. Now that she is a little older she begins to ask questions. My little one is beginning to put the pieces together. She know barb is her cousin because Uncle is her daddy. She know Sam is her cousin because she know auntie is Sam mom.

My husband and I were not gonna talk about it with her. But the last time we got together as family, she made a joke about her and her cousin were gonna be like Sam and Barb. We didn't really say anything at that time.

Once we got home, we talked to her about how that's not possible. But she keeps asking questions.

I have never heard anyone being in this position themselves. How do I answer her questions. What do I say to her? Our other option is to take some time until we attend another family event.

Thank you.