V.B.
She's 5. You are 20 weeks along. That means you have another (roughly) 5 months left before the baby arrives. That's almost half a year.... which seems like a LIFETIME to a 5 year old.
I would just not dwell on it. It is. Just a fact. No need to continue belaboring the point. On the occasions when it is relevant, fine, but for anything else, I wouldn't draw any additional attention to the idea of it being a boy. Go back to saying "the baby" instead of "little brother" for example. She knows. No need to beat her over the head with it every time you mention him.
Just for a while.
In another month, you can refer to him as "him" or her "little brother." But for right now, I'd drop that detail from any references.
As time goes by, she'll become accustomed to the idea. And as long as you treat it as just one more fun detail about the baby and not something traumatic or "disappointing" for her... It'll be fine. She will come to see it the same as you, whatever that is: a disappointment; or something fun, new and different.
But it's still a LONG way away for her. It seems like right around the corner for you, probably. But, don't you remember how it seemed like eons between Christmases when you were young? That's where her frame of reference for time is.