Pissed off Sister

See if the venue will let you change your wedding date. Maybe explain the situation to them.

Maybe it's just me, but I don't see the issues with weddings being schedule at least a month apart. I could see an issue with her wedding being in the same month, with just a week or two in between, but she had some valid reasons for picking the date she did. She allowed her sister to have October because she knew it was already hers. That's the issue I see with this, she was respectful of her sister's choice and yet her sister ends up copying hers.

IMMEDIATELY STOP SHARING DETAILS. No colors, favors, band, etc. Tell venue not to share with her. Tell your mom not to share.

In my circle right now, there are a TON of us getting married in 2020.
My matron of honor late april 2020
One of my bridesmaids, late may 2020
I'm Late June 2020.
That bridesmaids sister is september 2020.
My matron of honor and I are very similar people, with very similar styles. She was discussing what she wanted and I was super bummed for a day, everything she was saying was all stuff I was planning on doing! Ah! I know my bridesmaid and her sister had some issues picking different colors and caterers. My matron of honor and I both wanted the same dress for a little while. If there is clear communication and love involved, all of these things work themselves out. It becomes a problem when you decide to fight battles that are not worth it, and work against each other instead of as a team. She should have communicated with you that she was going to put a deposit down, and asked instead of told, just as you should have done when you put your deposit down knowing she still wanted to check the place out.
I agree with you and I would be upset, it would be weird to have the same venue, and you don't want people comparing them! But it sounds like this was a shouting match between you two rather than a loving conversation. It sounds like you both need to take a deep breath and then come back to the problem. If I had addressed my matron of honor when I was PMSing, tears streaming down my face into my fiances shoulder, devastated like never before about that stupid dress, it would have been a mess and I would have been in the wrong, because the truth of the matter is it's not that big a deal. Yes, I said it: her buying the dress I wanted was NOT a big deal. When you're inexperienced in something, would you agree that everything becomes a bigger deal? A toddler being asked to share will of course throw a fit. Failing your first big test hits you way harder in third grade than after you have dealt with it in high school. Your first falling out with a friend is always the worst one and hits SO hard. You know why being a bride makes people crazy? Because you've never done anything like this before, and EVERYONE has a different rule book, but you only get to do it once. We aren't experienced in this, and everyone we turn to to ask if we should be upset or not has a different answer, we have to choose for ourselves, and it is SO easy to choose anger, because we only get one wedding day, and it makes us instinctively selfish. Being a bride is like starting at a new job, and having these conflicts are like deleting an important file. Ah! What do you do! How do you handle this! Who do you turn to! The first time, you will probably mess up the fix, and be way angrier than you need to be and short with those around you. The 15th, you'll be fine. The problem is, you only get one go at this, you don't get a 15th time. So you need to learn to just take a deep breath with your sister, and know that none of this matters unless you let it. Do not look back on these two happy occasions with grief because of how you handled situations like this.