Get pregnant accidentally on purpose?

My sister, NOT ME (need to clarify because i don’t want all of you bitching at me, i know it’s not the right thing to do but i told her I’d ask on here) is trying to get pregnant with her fiance. He doesn’t want to try but he’s not against it (if it happens it happens mindset) and she’s like ready ready and wants to know how to get pregnant and make it seem like it just happened? She’s 24 so she can make her own choices. Any experience or know someone who has done this?