Advice... Am i being irrational? Long

Sillylttleone

Background: My husband and I got married in May of 2010. My brothers girlfriend was jealous so she got "accidentally" got pregnant right after. Their son was born last April. They are not financially capable of raising a child. Ever since then my parents have catered to their every beck and call (clothes, babysitting, anything the kid needs) and the baby is all I ever hear about constantly. Mind you, he is super cute and a great little boy but it gets to be to much sometimes. My husband and I had planned and are now having our own baby. 

Situation: My mom told me to coordiante plans with myself, my bestfriend Gina (who will be the babies godmother) and herself. Doing so, I asked my mom to go to brunch with us just to catch up considering we haven't seen each other in a while. She instantly said no, that she had to watch my brothers baby every Sunday. I asked if she could try to see if his girlfriend could find someone else, or if she could double check to verify that he was working as he doesn't always and she said well maybe, can i tell you the week of if i feel like it? I feel like this is being super selfish and inconsiderate that she wont even try. When I brought up to her nicely how I feel, she said that I'm causing drama and that I was being unreasonable asking her to try to find someone else as this is her responsibility. I feel like this is unacceptable. Am I being irrational or do you think its to much for me to ask of her?