I am having difficulty showing and explaining how to be considerate and thoughtful of other's feelings.
How can I teach a child to be consciously thoughtful and observant of other people?
What are the problems of being inconsiderate to others?
Humans learn by three methods: imitation, gain, and loss. Let’s discuss each one.
First, remembering that a child who is twelve is entering puberty and a very selfish stage of development my help you to maintain patience when you otherwise might not.
Even kids who were previously incredibly considerate, can change and shift dramatically and quickly during adolescence. There is a lot going on in their brains and making connections that use to happen quickly and easily becomes temporarily slowed while they start focusing EVEN more on fitting in with friends and being a part of the accepted and "cool" crowd.
Not surprisingly, hormones play a part in this, but only one part. Brain development is occurring now at a rate similar to what occurred during toddler-hood. Part of the forgetfulness, disorganization, sleepiness and moodiness we see in teens and preteens is also due to changes in the brain as an incredible growth spurt in synapses and development of a myelin sheath on the frontal lobes occurs (for more detail and information on pre-teen and adolescent brain development).
The fact that there are reasons and explanations for a lack of consideration does NOT excuse the lack of consideration though. It just tells us parents (and teachers) that we are in it for the long hall. As others have said, remind your child of things like "The Golden Rule" and the same rules regarding consideration you have always used while you continue to be a good example of considerate interactions and living.
Specifically for a pre-teen, you can take on the role of guide. Tell her that as her parent, it is your job to teach her how to maintain good friendships even into adulthood (teenage friendships are GREAT, but they are a little different from adult friendships) and that being considerate of others is a part of that. Friends are the ultimate motivating force for most kids at this age. What she does at home with you and with her siblings is practice even for what she does with friends - she may not believe you, but point it out to her anyway.
It is also important for you to know that because of where she is at developmentally, it is very likely she will only hear and take your advice if she knows you have heard and understood her first. When she finishes speaking, paraphrase her and say, "did I understand correctly?" When you get a yes, you can insist she do the same for you and expect that her listening to you will be genuine. Besides, listening well is a part of modeling consideration anyway.
If she says or does something to hurt your feelings, state how your feelings are affected (or how you guess the feelings of the peer or sibling may have been affected) and ask a lot of questions.
For example, "Why would you say that when you know it will hurt my feelings?" When she answers "because I'm angry (although she'll probably use a different word for angry)" You can respond with, "Oh, well, wouldn't it work better just to tell me that you are angry and why?" Ask her what made her angry and even if you don't give in, you can let her know you understand by hearing her out, paraphrasing her statements and feelings. THEN discuss your concern, "Calling names is a good way to push friends and family away instead of figuring out a solution. If she does something inconsiderate (like walks into the room where you were sleeping loudly talking and laughing and switching on the light) you just have to figure it is a teachable moment and talk to her about it. Try to keep your voice fairly calm and nearly emotion free but state how you feel and what you wish would be done differently as you would expect her to do toward you. "Wow! That was an awful way to wake up from my little nap. I'm feeling pretty frustrated about it. I hope in the future when you know I wasn't feeling well and trying to get a little rest you will be quieter. Can you please go be in another room while I try to get a little more sleep?"
At this stage of the game, expect to do more listening than you do talking; more questioning than answering. It will help her trust that you really are on her side. Model considerate behavior by you toward her even when you are frustrated and make you her ultimate guide instead of the typical parent-enemy. Besides, by listening, you also often get insights that help you guide her more precisely anyway.
First of all, there's a definite age before which this point simply cannot be made. How old a child are we talking about?
Assuming that we're talking about a growing child, say 4, there's a very simple phrase that I used to great success: Treat other people the way you want to be treated.*
Keep your eyes peeled, and you'll have ample opportunity to support both the giver and receiver sides of this statement. All things being equal, it should only take a small handful of interactions before seeing results.
Example: Marceline snatches a toy from Finn, Finn throws a brick (megablock) at your kid. "You wouldn't like it if Finn took your toy would you? He didn't like it either."
or the flipside "Hey look how happy Finn is that you gave him some of your goldfish! He likes that! You'd like it too, wouldn't you?"
Truly tho, being considerate in the way you describe (to be consciously thoughtful and observant of other people) is a later concept. 7 yrs old, maybe even older depending on the kid.
A younger child (4yo) understands Mean and Nice, and they get how to play well with others. They will get that sharing their goldfish with the other kid is nice, but will have no idea how to be observant that the other kid didn't get goldfish and then be thoughtful enough to share them.
Unfortunately, unless that Mean and Nice groundwork has been laid as a toddler, you're probably going to have a hard time promoting them from Nice to Considerate when they're school age.
*otherwise known as "The Golden Rule"
Be a living example and do it with naration. In other words be considerate in front of your child and do it witnural narration. Don't say (to your child), do you see how I was considerate, rather say (to yourself), I am so glad I noticed that so and so was feeling down and I tried to make him/her feel better or I am so glad I was able to be polite and say excuse me to that nice lady. Your child will learn from your example. As well, when your child is inconsiderate do a similar thing, saying (to your child), I wish you had been able to ... it would have made you feel so good.