He doesn't wanna know Gender but she does? (Please read bottom b4 voting & commenting)

My sister is wanting to know the gender of her baby to make things easier on preparing. But her man is refusing and wants them both to be surprised at birth. She is torn on what to do because she doesn't wanna go against her spouse but she feels it's unfair to her as well. Would she be a horrible person if she did the sneak peak behind his back and just didn't say anything? What would you do in this situation because he will not budge and she says she just wants to bond better and prepare better. She asks my opinion and I honestly don't know what to tell her because I won't tell her to go against her husband but at that's same time that's my sister 👭 and just wanna see her happy. So I'm wanting some opinions on this situation in order to give her the proper answer. Please no bashing her she just wants to please him but feels it's unfair to her thoughts on the situation and he won't listen.