I cry easily too, and it annoys me.. I don't want to cry, I am trying to be firm and get my point across and then the stupid tears start and I feel like I've lost my self respect, or they think I'm putting it on to get the sympathy vote.
Not sure about you, but I never used to be like this.. when I was younger I don't think I was like this at all. I'm 31 now and I guess I've noticed it more and more over the last 7 or 8 years.
I just fell out with my family as I said in the nicest possible was that we didn't want as many visits. They live abroad and want to stay for a few days every 2-3 months and me and my partner really feel strongly that we don't like it or enjoy it. I got a lot of criticism for it, but I'm glad I spoke it and stopped being a "people pleaser" for once. It's taken me weeks getting over it, feeling guilty etc.. but I keep having to say "It's my life, not theirs, I need to stand up for what makes me happy".
The worst part is, the more you dwell on it, the worse it seems to get! It's a vicious cycle.
I've also noticed people don't understand as I'm very social when out, been part of large groups etc, but I like my space, my own time. I've dropped countless people who wanted to be my friend because I felt they were asking too much of me and didn't it when I said no to things.
Anyhow, I feel your pain! Especially on the thick-skinned sister.. gosh, mine makes me feel like such a freak sometimes, despite being successful in my life so far.