Awww Sabrina I can see why it would irk you a bit at first. My bff got engaged about 2 months after me, and she got married this past June ( so like 4 months before me). At first I was a bit ticked by that but then I realized I prefer it that way...her wedding is over now, and I still have mine to look forward to. I think she's the one who's secretly jealous now because she was so sad when hers was done. Like Nadine said, don't worry about your mom not finding time etc - your wedding is still awhile away so she has plenty of time to do it, and she committed to you first so she won't back out! She's your mom - she'll make it happen.
I think it would be super fun to be engaged to your sister at the same time - now you guys get to share the experience together! Share planning resources, info, and excitement! I've been waiting for my MOH to get engaged forever and I really wanted us to be planning our weddings at the same time but her stupid boyfriend hasn't proposed yet Makes me sad. Try to look at the positives because there ARE some, I promise! It may not seem like it now but once you sit with the idea and get used to it, you'll be OK