William, that sounds like a tough situation and I wish you all the best as you work on your relationship with your sister.
The best advice I've heard about maintaining strong relationships is that successful relationships are 80% give and 20% take...for both people. Because of cognitive biases that distort our perceptions of the balance of efforts and rewards in relationships and because of the inherent difficulty of connecting to another human being with her own complexities and idiosyncrasies and hopes and fears, it's easy to start to feel like people are making unfair demands on your time and energy. But it's only by acquiescing to those demands, however unfair they may seem at the time, that you can start to bridge the gulf between yourself and another person.
The very fact that you made this post, rather than just cutting your sister out of your life, indicates that you believe this is a relationship worth preserving and improving. Like anything worthwhile in life, this will require hard work. A lot of the things that you seem reluctant to do - expressing interest in her life, "putting her in the spotlight," maybe reining in the sarcasm around her and possibly being a little bit more careful when discussing lifestyle and money choices - while difficult, are things that loved ones do for each other's sake in happy and healthy relationships.
It seems like you find it difficult to connect with her because the two of you are such different people. It sounds like she realizes this too, but is making an effort to build a bridge to you and include you in her life. I think that simply making more of an effort - offering sympathy, understanding, encouragement, support, and (careful and sensitive) advice - will show her that you care too, and will mean a lot to her. Try to understand the things that bring her joy and the things that make her anxious. Even saying something as simple as "it must be very rewarding to spend time with kids" the next time she tells you a booger or poop story will show that you're making an effort to see things through her eyes and build some sort of common understanding.
It sounds like you guys have some communication issues too. You say that you're bothered when she holds things in her back pocket and holds grudges against you, but honestly, it seems like that's kind of the same thing that you're doing with this whole bag-of-sugar episode. I think it would really help you guys to get to a point where you can talk about things like that instead of nursing them in the darker recesses of your hearts. "That day I asked you get a bag of sugar for me, I was really stressed out and I had a lot of things going on. I felt like you could have done me a big favor and reduced my stress level a lot, without really going very far out of your way at all. I don't understand why you didn't help me, and it hurt my feelings that you didn't."
Finally, I think it will help you to try to understand that not many people share the ideas about money that are common to those of us who frequent this website and these forums. I think a lot of us have found ourselves in situations where we've been talking about frugality, investing, early retirement, financial independence, and related topics around friends and been received with stony silence or outright hostility. It's natural for people to be sensitive, insecure, and ashamed about these sorts of things, and to perceive our enthusiasm and passion as an attack on their character. Try to keep in mind that talking about how good you are with money is a pretty easy way to alienate most people. With your sister in particular, I imagine she makes much less than you, and she might feel like you're rubbing it in her face. Try to be conscious of this and sensitive to it.
Good luck, man. Family isn't always easy, but it's one of the most important and potentially rewarding parts of life. Hope you can find a way to connect with your sister. (Or, alternatively, a way to get her out of your hair...)
