ooooooh, yeah!
Self-coercion- oh, yeah, is there ever such a thing! I think this is a lot of what we learn as children when forced to pick up the blocks or clean up our rooms or whatever. Eventually, kids internalize that voice, having learned to force thier selves to do things they don't want to do. Voila, self-coercion. And if feels horrible and gets in the way of us living the life we want.
And I think people lose track of the ability to know what they want, through this same process of coercion. This is a form of coercion damage, imo. The ability to identify what a person wants in any given situation is essential, to be able to find common preferences, to find good solutions, or even to make good decisions. Have you ever dithered over what to order off of a menu? What to do today, when today nothing is scheduled for you?(hah! if only...most parents don't have the luxury of a day to themselves!

But I highly recommend taking one, whenever you can arrange it!) Felt guilty about doing something that you just want to do for yourself? I think we all know how this feels.
I recall a big breakthrough for me, in my thinking, when I realized that it is ok to want what you want, and it is ok to get what you want. That I could figure out ways to get what I want without hurting anyone else in the process, and even to help them get what they want, too. Yahoo!
In the nasty relative scenario- what would the hypothetical person really want in such a situation? First, defense of their child (if there is a child involved). A parent's first responsibility is to their child/ren and their own self. I don't think a person can go wrong, when acting in their own best interests (which would include others' best interests as well, it seems to me, it's all tied up together) and this is the best motivation for children to act from as well! A person could speak directly to their child and reassure them that they have done no wrong, that parent does not agree with the nasty relative, and how about we go get some ice cream (or whatever). If the child is not directly involved, a person might weigh their responsibility to this nasty relative versus their responsibility to their own self- would it be best to extricate one's self from an altercation in a pleasant way, perhaps helping the other person save face and beat a hasty retreat, if this person is not open to new theories? A swift 'I see we do not agree about this. Please excuse me' might be all that is needed. Or, if one is articulate in times of such stress, one could reel off a well thought out defense of their position/theories, and perhaps the nasty relative and the person can have a discussion and both learn something.
I think that the ability to identify what one really wants is essential in taking one's self seriously as well as taking others seriously- I want to finish typing this message, but if a child needs my attention, even though I might heave a mental sigh at having to leave in the middle of a thought, off I would go because my priority, what I really want, is to be available to that child when child needs my help and attention. I know I can come back to this later, whereas a child fobbed off with promises of 'later' 'just a minute' gets a clear message about mom's priorities. Let me hasten to add that if a parent is engaged in something that is not easily interruptable (though these sorts of things can be done at times when there are others available to help children) or that they are really absorbed in or would like to reach a spot that is more easily interruptable, a child is often willing to help a parent out and wait for them to do so- especially if they know that if it was something really important (by the child's lights) that parent would become available immediately.
I think that self-coercion has some things in common with self-sacrifice, which is another aspect of coercion, though they are not exactly the same thing. We do well to learn to recognize when these things are happening, though, because then we can start to find good solutions to these problems.

Thanks for the discussion!