Your opinion on boys dating older women

So ... My brother who is 23 is dating a woman who is 45...
She's almost the same age as our mother.
I'm so confused.
I don't know what to think right now.
I mean, I want what's best for my brother, but I'm not sure that this is the best for him....
I also feel a little bit jealous of her..
my brother was like a father to me.
We used to be best friends and now, we barely spend some time together. He always needed my advice and he doesn't anymore, he always used to stand up for me and he doesnt :(..
maybe that's my problem...