Is it normal to be super obsessed/jealous of future sis-in law?

I am 21 yrs old, 5'4, a little overweight now thanks to the side effects of medication I'm taking(I'm really ill at the moment and used to be thin just a year ago), I have long medium brown hair, scarce brown eyebrows, light brown eyes, fair and clear skin and slightly crooked teeth.

She is 16 yrs old, 5'9, average weight, she has long dyed jet black hair, perfectly shaped jet black eyebrows, bright blue eyes, pale skin and the perfect straight smile. The only thing I ever notice is how she has a lot of acne scars but she covers it so well with her blended face make up that I rarely notice.

Every time I see a photo of her, I want to look just like her as she looks super pretty. I've become so obsessed that I am now buying products she owns. Whenever she has a product out in the open, I look at it and buy that exact product and now I'm thinking of changing my appearance.

I've always had an admiration for people with jet black hair and always wanted to try it myself and never did but after seeing how amazing it looks on her, I decided I'm going for it.

Before you say something like, "you have too much time on your hands", that's not true. I am an extremely busy person and don't spend hours on the computer looking at her photos nor do I even see her often as she lives far from me. I hold a job and work a lot as well as visit doctors frequently so I'm always busy but for some reason, this occupies my mind regardless of what I try to do/think about.

I want her hair, her eyebrows, her skin shade, her teeth, her height. I don't think I'm below average as I do get compliments enough from both genders and my fiance is certainly above average according to many women he knows but I think I'm just at average and this girl is way above me. His Mother also always says she looks gorgeous in all her photos she posts on FB but never writes that on mine. Instead, she'll just write "lovely photo dear" and that's all I get. From this, I assume it's because her daughter is prettier than me and I'm hideous next to her.

The other day, I showed my friend a photo of my fiance's sister and me together and just "jokingly" said something like, "I look so horrible when stood next to her as she's gorgeous" and my friend then told me "why would you say that when you're obviously prettier than her?". I don't understand if that was serious or not but I let it go.

Also, when I first met my fiance's sister 6 months ago, my boyfriend told me that his sister told his Mother that I was "very pretty" but I'm thinking she just said it to be nice or it was used in a normal context and not as a compliment.

I know this is so long but thought I'd share everything.

Has anyone experienced this sort of jealous for anyone before/is it at all normal??