Am I being unreasonable? MIL asking for $

So my husband makes good money and has been helping his mother financially by giving her 500/month, and has been doing so before we even met. It never bothered me because thats his mom!

I asked why hes the only one helping his her since he has a sister who makes good money too and he said because when they made the arrangement his sister had 2 little kids to care for.

My husband recently received a promotion and a raise and when he told his mom her first question was how much was the raise and he told her, which is fine its none of my business what he decides to tell her!

Flash forward to today, she called him crying that she needs more money...

But now we are 8 months pregnant with our first baby, we just got a new house, I’m going on Mat leave for a year where my pay will be significantly cut and we are planning on me being a SAHM since he got a good raise! We were talking about ways to save money by going from 2 cars to 1 etc so we are being careful with money right now.

My husband feels stuck and feels he needs to give her the $. I try not to give my opinion as this is his mom and technically his money but I just find it extremely unfair that his sister who now has grown kids and a beautiful big house is not helping...

I feel selfish saying this but we need the money as well now raising a baby.

What would you do in my shoes? How can I support him giving her more when I want to make sure we have a comfortable life ourselves first...