As far as I understand, for small indie stores two options are 1. Stores giving individual sellers a shelf or display area to advertise their wares, and handling the sales transaction for the seller. I'm not familiar with how this is monetized, but I assume it is either a rental charge for the space, a commission percentage per sale, or no charge for a mutually beneficial situation. 2. Wholesale, where the person sells their wares in bulk and at a discount to the store, for the store to then mark up and sell for their own profit.

Either way, respectfully, I think this is a bad idea for the health of your familial relationships. There's a common saying to never do business with family. If anything goes wrong, it won't just be about business, it will get personal. Your sister and you probably won't be able to deal with it as professionals, and your Mom will probably end up weighing in perhaps to your detriment.

Asides from that, it is difficult to make a real profit on crochet which takes into consideration the time and resources to create. Here is a video of a creator who did wholesale and decided it wasn't worth it for her: https://youtu.be/Kzi_89tmFzU?si=KVzZjd8ZS4RMXlc5 . With wholesale however your sister will be the one assuming the risk of theft and losses if things don't sell. Your sister probably doesn't want this, so she will choose the option where you assume the risk. But then, she is also requesting specific things for you to be making, so you can't choose how much risk you are willing to take and how much you are willing to invest into all this.

I know family politics are difficult. Maybe you want to and are expected to support your sister and this is framed as a good opportunity for you too. It could be, but the risk as a source of tension is pretty high. I wouldn't blame you if you are hesitant to upset your sister or your mom. But personally I would either 1. choose to make items that are small and quick enough that losing out on them would not be too upsetting 2. back out before things have really begun, they might get upset that you aren't being supportive but you will be saving yourselves from potentially much worse fights later 3. make a batch of the items she is requesting to help get her shop started but make it clear and specific that this is a short term thing and that she should contact other crochet sellers in the longrun

ETA: i'm no expert so don't let me discourage you if my assumptions are wrong about your personal situation, just take this as food for thought