Just wanted to say that as someone who has battled with my weight forever (my family given nn was Triple T (Tubby Two Tits)) and who is your size, that when I got down to 52 kgs, I was stoked! lol So I dont think you should be worried lol
I agree with the other girls.

But wrt to feelin glike you should be thinner and interesting thin g I found out whilst I was very thin was that if I had've got down to below 48kgs, for my height if I went into hospital, legalyl they couldve kept me there until I put on weight. 49kg and below for a 5'4 girl is considered underweight to the point where you are deemed anorexic!

Maybe you should just tell your sister that her constant comments on the size of your thighs is really boring and could see start reading the newspaper to have something interesting to talk about ;P