Am I allowed to demote my MOH?

Hi, hello, how are ya. My wedding is in May. My MOH is sick (had to move in with her parents for care, walking and dancing at a minimum). My MOH also recently got engaged to someone she was dating for 3 months and totally forgot that I'm getting married (a year before her) (to my love-of-my-life-of-8-years) and oh yeah, she's agreed to be my MOH. Anytime I talk wedding, it's immediately turned to her - her sickness or her wedding. All of her other friends have given up on her, and she consistently tells me how grateful she is that I'm there for her. But, like, she can't do shit for my wedding. She didn't even know what weekend I said (1000 times) worked for my shower/bach party (lotta out of town girls) and it's MY BIRTHDAY. I'm just thinking, ok, you're dealing with a lot. But at least pretend to care about me. 

I asked her because I didn't want to burden my little sister who is young and broke, and my friend seemed really enthused to be a part of the wedding. I feel like the only reason she said yes was to be like "look at what a great friend I am!!!" But if she's going to suck so bad, I'd rather have my sister standing next to me. I'm going to spend holidays with my sister forever, she'll be the aunt to my future babies, etc (I KNOW I should have just picked her, but I wanted stress at a minimum, and like I said, I didn't want to burden my sister.)

She also compares/competes now - telling me how much her ring cost, telling me how much her florist is, etc. and then asking me about mine. She is also stealing just about every idea I have and then executing it more and better and grander. She has a bit of a Kim Kardashian attitude where she brags about things she didn't exactly accomplish. When I confronted her about how much all this bothered me, she said "I thought that being engaged together was something fun we could do together!!" and then basically stopped talking to me (including getting back to me on real wedding plans like - do you want your hair did bc I'm making appointments). 1. I'm going through this engagement with my FH, not a girlfriend, and 2. What the what?!

She's been out of commish from the real world for about a year and a half now; she does not do normal human people tasks like working, doing laundry, etc. Her mom does all that. I know that with her illness and her life situation, I am not a priority, and I'd really just like to demote her to regular bridesmaid and promote my sister. Is this crazy talk? I feel like this way, she can focus her herself and I can actually have a nice shower/wedding/etc without worrying if my MOH is neglecting me or trying to one-up. She won't take this news well, but at this point, am I totally evil if I don't care?

On top of all this, my FIL is sick, money is tight (we're paying for the wedding, but out parents keep adding expenses that we Have To Have.... do we get a honeymoon, I dunno), and I started working a high-stress job. I feel like I need to remove any excess stress. 

-Homegirl is making me turn into a Bridezilla, gross