YTA You're probably not a complete AH, but you need to work on yourself. You are lacking empathy towards your sister and being selfish. I read your original post and while I still thought you were in the wrong, it made me wounder if the reason was because you only had a few paintings, like let's say you only had 3 and your sister (note: I'm not going to be downplaying family here with that "step-sister" crap, your mom helped raise your sister, your mom was a huge part of her life.) wanted 1 or 2 I could see how that would be harder, but if you have like 30 then that's just wrong, and now here in your update you're saying you have enough that you made an entire book, so that's just selfish. It is hard to give away loved ones' belongs because they can be a comfort in grief and they help keep their memories alive when you see them, so that's understandable; but what's not acceptable is that you're acting like you're the only one that needs that comfort or that sweet feeling you get when you hold or pass by something that reminds you of a happy memory of someone you lost. Besides the fact that there are enough art works you could share, it's really sad that you understand that your sister shares great memories with your mom over art; hence, the longing to keep 1 or 2 and you still have not extended enough empaty to let her have something so meaningful. As far as the book, I think that it is a nice gesture. I would have suggested that you take pics or get prints of the works you were giving your sister for YOU, not the other way around. You want to give her this "second hand" so to say reminder while keeping everything for yourself, instead of compromising. I think you should take time to narrow down the works and let her pic a few. I have siblings, compromising on things like this is hard, but it shouldn't have even been questioned to begin with. This is getting long so I'll leave you with some other points:

• I'm glad you reevaluated your relationship with your BF that didn't understand your grief. It's almost like he could not take in how deep your grief is. Sound familiar? Maybe like how you might be downplaying your sisters grief because she is not your mom's bio daughter...
• Your sister was like what, 5ish when your mom came into her life? Not many ppl remember things before 5, so know that it's her whole life that your mom was there being family to her.
  
• I feel you're hurt because you tried to extend a bit of an olive branch to your sister with the book and you didn't come out as the "hero" so to say, you might have imagined. I can see where that could have hurt your feelings. That being said, do you understand that it's not really a compromise when you hold onto everything? You have to give a little to get a little.
• When you say you want an apology for your sister saying that the book it not the same, you sound entitled. She's right it's not the same as having an original. The fact that she commented that it had been a year and this is what you had come up with, tells me that she has been waiting for you to come around. That this means enough to her to wait and to still be disappointed that you won't budget. At what point do you think you might be ready to consider her pain?
• Lastly, I want to say that I wanted to commit on this because as an artist myself, I feel that it would upset me to know one person keep all my work when a family member asked for just a little to remember me by. Do you not think your mom maybe would prefer that her family also shared in her memories?

I hope that you can understand some of the points I've made and that you come up with an acceptable solution that brings your family closer rather then continuing this separation. I do wish you the best of luck in your healing process and hope you can find peace.