20 year sibling age gap?

My mother found out she is pregnant. I will be 22 by the time she has the baby and my sister will be 20. I have a 5 month old son myself. My mother will be 40 when the baby is born. She was not trying to conceive, she's actually really upset which sounds awful but her birth control failed. Her and her husband just got married last year, he's never wanted children. She's also worried about if my sister and I will ever be able to bond with our sibling considering we're both out of the house and I have my own family. My mother had us young and was a stay at home mom until we were 10 and 11.. she's built herself an awesome career and in general she's really terrified to have to go through raising another kid she feels like it's starting over. I'm trying to be encouraging but she's so upset. Her husband also is in a state of shock, considering her never wanted kids and quite frankly was told he would have hard time having kids because he apparently has an incredibly low sperm count. It's just a really weird situation all around and I'm trying to help her through and make her feel better in some way... does anyone have a large age gap between your siblings and how is your relationship?