explaining where babies come from to young kids

My SIL is ttc and she talks very openly and in great detail about the IVF process infront of my six year old. Now I have a six year old asking me about sperm. I've told my SIL that we aren't ready to have these conversations with our child and she told me I was wrong for not involving the 6 year old in the process. She feels it is her job to explain what is going on to our child. I personally don't think that a 6 year old should know all this nor that anyone else has the right to decide that for me. I'm happy to talk about her IVF process any other time,  just not in an open discussion with my child. How did you explain where babies come from to your kids and at what age is it appropriate to get into all the details?