Mum Is Pregnant and she hasnt told my twin brother?

I would just gently suggest to your mum that she can't hold onto a lie forever... especially not when the baby is born. Just talk calmly to her, and help he to see sense. If she doesn't, then you'll just have to try and keep up the deception, as you want her to be happy. But she can't lie forever... you have to try, but if you don't convince her, follow her lead and lie. Its all you can do.