Torn.. so torn..

First my sister was torn between me and Dave..but now I'm torn between my sister and Dave. Well, not really torn between them.. but I know my sister's upset that Dave keeps insisting that he spend time around me/us. Maybe he thinks that because I'm new and I haven't got many friends here yet, so he just wants to be nice and help me get used to the environment. But on the other hand, my sister is worried that there might be something more to it. Mind you, she's not being a jealous bitch because I know my sister well enough. She isn't the sort to mind if I hang around her boyfriends. But the thing is, I think Dave might be going over the top. Besides, Dave has been known to be that sort of guy. I wouldn't put it past him.

For example, he bought me a massive bunch of flowers for Valentine's Day - not a gigantic as the one he bought for my sis, but still big. And I knew my sis felt kinda awkward about the whole thing but she didn't know what to say. On top of that, at 1 am, my sis told Dave that he should go home soon cos they both have to get up early this morning and Dave agreed. But then he came out and sat with me, watching TV for another hour before he left. My sis was pretty upset about it. She says she feels like he's undermining what she says and she even asked me for a favour - to politely decline any suggestions that Dave makes for us to spend time together. Now I'm torn between whether I should tell my sis about the fact that he rang me earlier asking whether I wanna come over to his place and play on his XBox. Or not. Like I want her to be aware, in case there is more to the eye, but at the same time, I don't want to cause her any worry. Cos I know she's frustrated about the whole situation right now. So do I tell or not?

I just don't know how to decline his invitations without seeming rude or as if I dislike him. Cos then he might try even harder to get me to accept him.