Hey there :)
Straight off the bat, let me say that you come across as highly intelligent yourself - everything you've described of your predicament is written from the vantage point of someone who thinks deeply and sees herself clearly.
Which is why I know you won't begrudge me for a few suggestions.
Having dealt with envy myself, I have noticed a few things:
1) It never really goes away...for as long as you look at a person from a distance. Like you, there used to be this one girl I felt resentful towards because I thought she was smart, beautiful and accomplished. I wanted to believe that I had my redeeming qualities but being in her presence inadvertently led me to make unnecessary comparisons.
However, she was on my course at uni and we started getting to know each other better. For at least a year, we were 'friends' but I felt passive-aggressive deep down. In time, we grew closer though and I learned of her own deep-seated insecurities. She showed me how her ears stuck out something awful - which she hated but I'd hardly noticed because of how well she covered it up. I saw that she was smart but that she had her weak moments too, like any of us. She was still pretty, had a great social life and nicer pictures on FB but the more time I spent with her, the more I saw her as an ordinary human - one with bad hair days, wardrobe malfunctions, clumsy habits...the lot. Today, we're actually best friends. Looking back, I'd never have believed it was possible.
Sometimes, it seems to me that we put people up on pedestals and see them as being extraordinarily better than us. So even if we don't want to be like them, we feel envious. The best way around it is to really get to know a person. See them for who they are. The truth is very freeing.
2) Learn to look away from them and towards you. I don't know your story but one of the reasons I fell into the trap of comparisons is because I was compared a great deal during my childhood. Amongst other things, my mother used to compare me unfavourably against other girls my age - in terms of looks, personality and intelligence. This set me back a little, emotionally... (haha).
Knowing what types of triggers lead you to feel and think along certain patterns shall help you address them. Please know that no one can make you feel insecure or like a lesser being, unless you allow them to. So begin building your own sense of esteem. It will take your focus away from something as unhealthy as these emotions of resentment towards your bf's sister and towards an investment in yourself. So, for e.g., take up a new hobby, volunteer, pick up a new skill. As your life becomes richer, her life will have less of an allure.
3) Determine for yourself that you will master this emotion. In life, we will always come up against those who are superior to us in some way. There's always something. They're taller, stronger, more assertive, better dressed, you name it! We cannot live our lives feeling inadequate every time someone 'better' comes along. So determine within yourself that you will not entertain envy. As you feel it rise up in you, remind yourself of how counterproductive it is. How it is based on false premises. How it is a product of X, Y, Z experiences. How you have plenty going for you. Speak to yourself in affirmations - be your own best advocate, internally. When you have a strong understanding of yourself and a generally good relationship with YOU, then the external world becomes far easier to navigate.
Trust me on the above...they're the hard-won lessons of personal experience :)