I think all parents will have this experience to some degree, a little or a lot, as their children grow toward independence.
And there are always ways for parents to change their own attitudes and behaviors that will make family life easier. So I'm really glad you have asked, and you have been given some great advice in these responses from experienced mamas.
Let me share the thing that jumped out at me from your words, D.. You said, "We will say something like straighten up…." I noticed there was only a demand in the way you stated that, and a rather disrespectful demand, at that. Your statement may not have been accurately descriptive, but if that's the way you correct your son, may I suggest that he might NEED more respect, empathy, and compassion from you in order to learn how to "do" those positive things in his own interactions with others.
There are many ways to make that request that will show love and concern, and demonstrate your respect for your child's feelings. Instead of "Straighten up!" you might try to provide more information about how YOU CARE for your son:
It might sound more like, "I see you sitting slumped over, and I worry that you feel sad or tense inside. If something is keeping you from standing up and feeling positive, I'd like to hear about it." Or you might be feeling, "I worry about you, and about how other people will see you when you slouch like that. Have you noticed how positive and successful people learn how to use their bodies to show their strength to others? It appears to me that people who slouch don't earn as much respect."
Even learning to routinely say "please" when you speak to him would be a positive move. It sounds as though your son might have a very active internal "compass" around the principles of fairness and right and wrong, and he could reasonably feel that you don't live up to his principles, resulting in diminishing respect toward you. I've seen parents tighten up with their own fear when kids begin to rebel, and become more harsh and less respectful.
Of course they care intensely, or they wouldn't get so uptight, but their love doesn't show in their behavior. And it only makes matters worse. Rebellious children need to continue to hear plainly that you love them and care about who they are inside, even if they are being difficult on the outside.
YOU are the adult here, and the more gently and consistently you can model that, the better your family life is likely to be – for all of you, including your young daughter. Look at your family rules and boundaries from your children's point of view. Some correction may be absurd or unneeded, and if so, would best be retired. For example, nagging a kid about standing up straight might be more of a control issue for you both than a useful rule. Identify and stick with the ones that make family life better, safer, more positive, and more compassionate for all of you.
My best to you. And please don't overlook the possibility that your son might do well to have a psychiatric evaluation. Some children are wired in ways that they don't like and can't help. I have a younger sister who didn't get the emotional and medical support she needed when she was young, and suffers with the consequences every day. And of course, the rest of the family still suffers along with her.