Well first of all what a load of crap regarding the morning sickness theory. I had not a day of queasiness with my first DS however this pregnancy I was vomiting 4x a day and had to be medicated, also a boy
Secondly, your SIL's quote about son's vs daughters is pure ignorance. Whether my son's get married or not they will always be my sons. No-one can change that. If I was her I'd be careful what she says because there is no guarantee she IS having a girl. It's hasn't been confirmed.
Thirdly, I don't have any tips as such because I am completely thrilled to be having my second boy and have no desire to have a daughter but if you change your perspective to believe that our children choose us for a reason and not the other way around it is actually quite beautiful to think they chose us to learn their life lessons. If a girl/boy wants to incarnate with you as their parent, they will. I have always known having a girl was never on the cards for me and I accepted that a long time ago because I trust the universe has a plan for everyone. I tell myself every day what a privilege it is to be the one responsible for shaping good men because the world needs more of them.
Don't be hard on yourself and feel however you need to but if you take a step back, you will see how it really doesn't matter what gender our children are, they just want to be loved for who they are xx