Sianesse isn't thinking straight.
Rebecca didn't tell her when asked that she would pick Luke, because she didn't know the picking order for sure.
If she had told Sian then Connor g would have found out. And if Luke was picked by someone else first, she'd have ended up in an awkward friendship couple. By not saying she had the option of picking Connor if someone else picked Luke first and Connor g, and us, would have been none the wiser, she'd just have said he was her preference.
I think Luke is wary of her. I don't see there is going to be romance there. If someone better comes in, a more alpha Male who she thinks will be popular, Rebecca will drop him immediately.
Rebecca is pissed because Sian is calling her out on her behaviour and making her look bad, so she's trying to twist it that the issue is Sian is jealous and wants to get with Luke and feels Rebecca is a hinderance to her. When we all know that's not true, it's why she shouted last night "for God's sake you can still get to know him" and kept trying to make that the issue to deflect from her own behaviour.
Rebecca is also lying, she lied that Sian hadn't asked her who she would pick, and we saw her ask her, she then lied that Sian was shouting at her, hence why Rebecca reacted as she had, Sian didn't shout at her.
She also lied to Connor saying he ticked all her boxes and going in for the kiss, when she then dropped him immediately when she saw someone else she thought would be more popular with the viewers.
She's not in there to find love, she's in there to boost her modelling profile and is looking to find someone to couple up with that will keep her in, and who she thinks the viewers will like.
As said, I don't think Luke will go there, she's a huge risk. Rebecca will bin him off without a second thought if she meets someone she deems better.