Sunshine has actually grown on me and I quite like her now and I don't actually like Ben so I'm not biased in that way
The only way I can explain it is it's in the same way that Vash nommed Freddie last year, he didn't dislike him and they did speak in the first week but he also annoyed him in a lot of ways before he got to know him better. Sunshine was VERY annoying the first week and she and Ben did clash although yes they did speak quite a bit. Because Ben seems to get very uncomfortable in confrontational situations and arguments, and SS did cause a lot of those in the first week, maybe she made him uneasy, and that's why he nommed her? I can't recall his exact reasons for it but that would be a pretty logical explanation to me.
I don't believe you can make friends with someone that quickly and if she was causing trouble, which she was in the first week, I can totally understand him nomming her. It was really unfair that it got brought out because he wasn't the only one to nominate her, 9 other people did too, she was obviously getting on everyone's nerves.