Pattycakes -- Oh how my heart goes out to you. Not so sure you're supposed to be alone, but maybe you're not supposed to be in this relationship/situation? How long after the mom committed suicide did you and he hook up? Popping up in my mind are themes related to saviour and martyr for you from your posts. It's hard to imagine why you would put all this on your shoulders. It's apparent it's crumpling you from the inside and breaking you apart piece by piece. If you don't do something to heal and protect yourself, it will manifest itself as something that will force you to pay attention. You already mentioned you are chronically ill. How much more ill do you need to be to take care of yourself? What would happen if you did leave? Dad would be forced to step up to the plate and take care of his daughter, or would he find another warm motherly body to buffer him from his responsibilities to her. (I'm sorry if that sounds harsh - I don't know him, but from your posts, it seems he's more than happy to turn away and claim it's the mother time she needs when he can't deal. His daughter needs him.)
I suggest writing out a pros and cons list and be really self focused with it. Pros and cons for staying, and pros and cons for leaving. See what pops up for you. You already have some of it started in these posts.
Get your psychic armour up. If you don't know how to psychically protect yourself, google it, or ask here. There are many methods.
Ground yourself regularly. Get outside in nature as much as you can. Walk daily. Get plenty of water, fresh fruits and veggies. Take epsom salt/baking soda baths if you can. Clear the energy of your house, daily if you need to. Smudge, crystals, sound. Mitchell Gibson has a couple of CD's specifially made to clear energy using sound currents. See if either of these reasonate with you, or anything else on his website. Soul Cleansing Spirit Attachment Removal
Explore EFT. Maybe there is a practitioner in your area. If you decide to stay in your relationship/situation, it may help you and your step-daughter. I sat next to the author of this EFT book for children at a conference and her energy was awesome. Amazon.com: Tap into Joy: A Guide to Emotional Freedom Techniques for Kids and Their Parents: Susan Jeffrey Busen: Books
Take care of yourself.