Am I wrong for being upset?

So sorry for your loss. You are not wrong for being upset.

From your follow-up responses, it seems like this kind of behavior is not new from her. My fiance and I had to have a similar conversation about a member of our bridal party. We knew they were flaky and would not be there 100% of the time or help when thing were hard. However, we asked ourselves if, 5 or 10 years from now, we would regret not having them as part of our wedding party. The answer was "Yes, we would regret it," so they are still a part of the party. We love them for all that they are, that's why we asked them in the first place.

If you only asked her out of a sense of obligation, then consider cutting her out. However, if you asked her because you genuinely wanted her standing next to you, then keep her.

I'm sorry to hear about your mother. People deal with grief in different ways. You have something positive to focus on with your wedding, but she may still be depressed. Think about how you might be feeling right now if you didn't have a wedding to plan? All of your energy would be on your grief. That's where she might be at right now. Give her the option of choosing to back out of being a bridesmaid rather than just taking it away from her. If she is unable to handle the responsibility, it should not be forced on her.

My sister-in-law lost her father in January, so she was unable to attend my first bachelorette in February, but she came to the second one in March. Her mother came to the bridal shower, but didn't want any attention drawn to her. I was informed of this when I tried to thank her out loud for my gift. They are both coming to my wedding, but have to have a "job" in order to come. In Judaism, a person in mourning can only come to a non-family member's wedding if they are working at it. So we hired my SIL's mother to watch my sister's baby. My SIL has already committed to being a bridesmaid since before January.

My brother has told me that she is still having a hard time since the loss of her father. People deal with loss differently, as I said, but that doesn't mean that we should disclude them because of the way in which they are dealing with it.

What are her "Excuses" for not doing things? Communication is key in any relationship, especially with so much going on right now in both of your lives. Consider that she is different than usual because your mother recently passed. Consider that you are as well because of the same thing, but you're also planning a wedding and that can change you too, maybe you aren't being yourself either. It sounds like you are turning to her and attempting to rely on her for emotional support, as well as support with the wedding, though you said you were not close before. Of course, your mother would want you all to grow closer together, but you need to offer up support to her as well, and realize that she is going through something difficult, you cannot drop a ton of responsibilities on her and expect it to be fine. I would tell her all at once what you would like from her for your wedding. It is something good to do right when you ask, but it sounds like you still have a lot left to do. List all the MUSTS ( ATTENDING bachellorette party, rehersal dinner, wedding) and all your wants (On X date you are doing diy decor and would love some help, X date shower prep, 2 or 3 bridesmaids to clean up rehearsal if its at someones home, etc etc). And let her know these are things you are looking for some help on, and would appreciate it if she can pick one or two, but understand if she cannot. When she said yes do being a bridesmaid, she did not say yes to these extra things unless you CLEARLY communicated that to you they were musts from the start. Some brides are like that. "Hey, be my bridesmaid and just so you know this is very DIY so that responsibility means helping, if thats not something you have the time for in your life right now I understand but please don't accept then." and thats fine if you communicate it. But it really should be "Hey, thank you for supporting me and my relationship, you standing with me is a symbol of not only our friendship, but the help you have been to us and me." It means buying a dress and shoes, going to two or three events, and dedicating the time you are able to help the bride, you do not have to drop everything unless you agreed to drop everything.
Communicate your expectations of her, your needs and your wants. Listen to her needs and wants as well. Meet her in the middle.

I'm not dropping anything on her. I didn't ask her to plan or throw anything. I just asked her if she was coming that's it. Me and my younger sister are planning everything.

I've have tried to meet her in the middle. Everything gotta be about her no one else. I bought her dress and paid for it to get altered cause she couldn't. I've only asked her to come to the events that's it. I haven't asked her to buy or plan anything.

I'm so sorry to hear you lost your mom. That is so, so hard.

It sounds like your sister has always behaved this way so I would change the one thing you can: your expectations. As much as you would like your older sister to be involved, she is choosing not to be so I would expect her not to be involved. As a bridesmaid and your sister, expect she will come and be a part of your wedding. You have done your part by inviting her to things. Now the ball is in her court for the future and you are going to have to find a way to move on without her wanting to be there.

I, too, have an older sister I love and who I know loves me. Since we were kids, though, her involvement in my life has been minimal. Her main relationships have been with her friends and our step-sister, not me. As much as I would like it otherwise, she has shown time and time again I am simply not a priority. So I don't expect to be. I did not invite her to our bachelorette (my wife and I had one together) and she may have been invited to our wedding shower, I don't remember. But she didn't come. She did come to our wedding and she did my makeup and I get to see her on rare occasions, usually at family gatherings. I love her for who she is. Would I like a closer relationship with her? Absolutely! Am I going to get that anytime soon? Absolutely not.

I know advice during a time of pain and grief may be difficult to hear but speaking as someone whose family relationships are not ideal, here it is: spend time with and value the people who are adopted family. We have friends who are like sisters to us, mothers, and grandmothers. Yesterday I attended the kindergarten graduation of a girl I consider to be a niece and the apple of my eye. Her dad is like my brother and he considers me his sister. This circle of adopted family are there for us, loving us, and supporting us as we are there for them. In fact, they made our wedding the beautiful event it was. Invest in those relationships and there will be a wealth of love and warmth for you and your fiance.