My "Little" Wants a Baby Sister

How do you explain to a 3 yr old that she is not going to get a Baby Sister. Especially when she says, "Please, I will be a really good girl and share my toys, please Mommy I want to be a sister". To which I answer that she is already a sister though her brothers are 15 & 18, which I full well know, does not count. So being the clever sort that she is she says "I don't want to be the baby anymore". Ok, then we will call you our big girl. None of this is helping and I just cannot come out and say that Daddy has had a vasectomy and cannot have anymore children, plus he is almost 45 and just too damn old to having more children. Add to that the fact that I did not want to have children, but am blessed to have her in my life. How do you explain that. So I simply and stupidly said "I am very happy to have had Mackenzie and now you, so really you have had a sister and are still the baby." My darling creature looked at me and said "Mackenzie was a baby sister, but she is dead with the angels. We need a new baby in your tummy." So at this point I say "Go ask Daddy!" Good Luck with that one honey!!

My brother is not married and does not have any children that he knows about. I have had one unsuccessful marriage and am on marriage number two. There are days I am more successful than others and keeping Juarez in my head probably makes for a crowded bed at times.
The thing I cannot understand is why my brother and I see relationships as disposable items. That is not true for all relationships either. He and I both stay in the relationships we have had since middle/high school, but do not do so well with the bonds we have formed as adult. My parents have been married for 35 years, though they were insanely unhappy for many of those years. Now I think they are co-dependant, but that is a whole new entry. Just things I am thinking about right now!