Urgent help with telepathic abuse!!!!

In order of the questions.

Yes, you can sever a telepathic connection by focusing on the connection and then either forcing your mind to close off to it effectively blocking him out of your head, or you could also make an energetic shield around your head to either keep him out or scramble what hes sending to you (this is a semi permanant solution) or changing your mental fingerprint via focusing on your consciousness and altering the ratio of your conscious and unconscious mind (if this person has access to your energy then you'd need to have an energy shield already in place to stop him finding out what your mind sounds like,) and last but not least, you can sever the connection by feeling it and tearing it out of your head...this method can cause some damage and is painful and doesn't always work, but it sends some very painful whiplash to the individual on the other end of the connection and is quite off-putting for them.

I'd suggest attempting all of these methods within a short amount of time and keeping shields up for as long as it takes him to loose interest in you. If there are alternate and compelling reasons for him to be manipulating you, then you may need to work around those in order to be rid of him.

Yes, a telepath can make contact via another persons energy. Its even easier if its through their mind (and since your very familiar with your sister you would have already had a mental bond with her of some sort, which is like a vip access to your brain pretty much.) I'd suggest temporarily removing or blocking your sister from your mind as well, as her consciousness would be tied to this mans in some way (most likely subconscious) and he could well also be manipulating her (especially if he wants her to himself and sees you as an obstacle to be removed...not saying thats the case, but anyone a manipulative as this individual sounds is probably going to have a master plan and a striking lack of empathy.)

Yes, manipulation via telepathy can be done against ones own will. It's actually considered a form of psychic attack and can be either very obvious as it is with you, or far more subtle (subconscious suggestions and blocking thoughts are the most basic methods, but manipulating motor functions and inducing obsessions or compulsions is also possible if someone is well trained.)

If you have any more questions I'd be glad to offer what I can.

I'm proficient in telepathy myself, so if this gets out of hand I could intervene on your behalf (though that would be a last measure and if he endangered your health in ways that you know to be severe.)

Hopefully your situation ends sooner rather than later. It's good to see that your able to divert your attention away from his mechanations, its a sign that he's either not very skilled when it comes to conscious manipulations or simply isn't trying very hard. Lets hope its the former.