buying lots of gifts or gifts that say "i love you" is bold and will often do more than raise eyebrows,especially if she is outside of your own family structure. paying more attention to the sister over the brothers may also arouse suspicion. I once was in a situation where I had been sleeping in the room with some boys,and noone even batted an eye. later,the sister got jealous and hurtfully requested that I sleep in her room too. so I went into her room and layed next to her after the boys were asleep. she was wearing pajamas as the room was chilly and I slept in my jeans and t-shirt. we both slept like babies. no "weird" stuff happened,much to my dismay. i was a teen at the time and might have gotten away with it. however,the next day i was told that it was weird that i should be sleeping in the room next to a LG,and so i was watched closely after that.
its not about what you ARE doing. its about what other people THINK you are up to.
if i was you,i wouldnt buy her a gift unless you have a justified reason,such as christmas or birthday. then you can buy something more special or expensive. i would be careful of anything that proclaims love unless you are trusted and well liked by the parents. cousins and stepdads may get by with it,but the guy next door may be viewed with suspicion unless you have a strong relationship with her family. (or unless you happen to know that mom or dad is open minded about such matters,which i would bet is rare) if you love her she will prolly figure it out in time as kids tend to read vibes and body language very well. kids can pick up on things that adults dont notice or try to deny.....