@Benja -
You are right in that it is more than being an ISTJ - he's being emotionally unhealthy. However, the manner in which he is giving rein to his unhealthy behaviors is very typical of an unhealthy IXTJ.
He will change, but you will have to change your tactics in order to get him to see the error of his ways. ISTJs are very prone to win - lose conversations, and it appears that he is giving into, or even pursuing this type of dialogue with you.
I would suggest that you seek clarity on the task at hand when he tries to delegate it to you. Get it written down and determine beforehand what entails a successful completion of the project. Then explain (kindly) that you don't want to be micro-managed and that he should arrange to leave you alone while you complete the task. Explain that once the task is complete, you will invite him back over to inspect and offer
constructive positive comments
and criticisms. He must engage in positives if he is to offer any negatives. Finally, if there is something wrong with the end result, he must agree to allow you to fix it. He must not undo and redo what he has given you to complete. Assure him that you will see it through to completion and to his satisfaction, but that the process of doing the work is yours once he delegates it.
Now, there may be something integral to the process that needs to be explained to you before you begin (e.g. proper method of surface prep prior to painting). Ask him to explain those things to you now, before you start. Then assure him that if you have a question, that you will come to him with the question. However, when you come to him with the question, it will be specific and he must only answer the question - he is not allowed to take over the project or to criticize other areas.
Once the project is completed, and he comes over and inspects it, it is likely that he will fall into the old pattern of wanting to be critical and redo everything. Don't allow it without calmly stating that for you to be able to work with him, he must follow what you've agreed upon. In a very nice way, you are telling him to back off, or he can kiss off a good relationship.
Speaking of that, make sure that he understands that this is important because what is really being sacrificed is a good relationship with his son, and the rest of his family. In a short while, you will be leaving to form your own life completely; when you do, it will one day occur to him that he would like a better relationship with you and your new family. This is important to ISTJs, so somehow you need to communicate it with him in a non-confrontational manner that lets him know that you want the relationship to improve, but you are not willing to lose yourself in order to please him. It must be give and take and he has to give you and others the room to be their own person.
As for the room, mess, whatever. In common living areas, pick up after yourself. Day in, day out, don't leave a mess for others. Do this, not for him or for others, but for yourself - be the kind of person that others look up to for this trait. However, in your room, it's hands off. That is your personal space and as long as you are not damaging the structural integrity of the room, it's not his call. If he doesn't like it, just keep your door closed.
Now, if you are truly beholden to him for everything - clothes, food, shelter, etc. (no job), then it is a little trickier. Here's the deal: He gave you those things. He doesn't want your clothes, he just wants you to take care of them like he would (or even better than he would) so that he feels validated and appreciated. The problem is that he has not really given them to you if he is attempting to dictate how you take care of them. If he feels that you aren't taking good care of them, then it is perfectly within his right to refuse to buy you new clothes after you ruin them, but it is not within his right to harp and nag because you aren't treating them in a way that is agreeable to him. Again, you will need to explain this to him in a way that gets his attention, yet in a calm and collected manner. Don't escalate.
I know this is tl;dr, but the last thing is your relationship with your mother and sister. It smacks of rescuing. Staying in an unhealthy situation for them is not a good thing. If they are in danger, they should get out. Otherwise, you must let their decisions be theirs, and make the correct decisions for your life outside of any attempts at rescuing them.
HTH