I'm not quite sure why English descriptions for affection degenerated so much over the centuries. There are Greek and Latin terms for more refined measures of affection and more than a few lasted into Old/Middle English.
Nowadays, its gotten very difficult to describe your relationship or feelings about someone.
"I love my sister" can go all different directions.
"I love my best friend" (right, right, we're all gay, time to punch the gigglers)
It can get particularly tense when explaining a good friend of the opposite sex.
"My best friend, she... what? No!! right, you die now"
"I have this girlfriend, no, wait, she's a friend of the opposite sex... no, I'm not bonking her, we go hiking.. .argh! I kill you now!"
Then there's "sig.other", "partner", "life partner" (blech), "room mate" (what?)
Perhaps the safest solution is to simply punch the lights out of anyone who laughs