I get nervous around my bf's sis?

My boyfriend has a really close relationship with his family. He has 2 sisters, a 25 year old sis and a 16 year old one. I get along with the young one because she's closer to my age (i'm 18). But I get so nervous around the old one because she looks at me as if she's looking at a child. I really care about my boyfriend (he's 21) and it's the first serious relationship that I ever had. Tonight we were going on a double date with his cousin and his girlfriend but now the older sister is coming too (such a third wheel). My boyfriend doesn't know that I get nervous around her. When she's around I usually stop talking because I don't know what to say so that I don't look childish. I am rather a fun colorful person, she's a real classy person. I don't know how I'm going to act tonight. We're gonna go in my boyfriend's car so I'm really gonna be nervous if he kisses me in front of her. Weird right? Any advice?