So im just wandering if it is ok for me and my brothers girlfrend to be really close?

[milton b]

Sure, you can never have too many friends and it sounds like she really likes you.

[jasonex]

of course, unless you dont like her. and no, you shouldnt call her sister. she probably does not have a sister and sees you as someone who could have been her sister

[Skinny Dipper]

Reading your question made me think you were his brother, lol. Of course its fine to be that close but unless your super close to her you might want to hold back on calling her sister.