If you want to buy the other female, you're going to want to either make a sorority (a tank of at least four females) or divide the tank into two sections, one side for each female. Even though your fish are sisters, female bettas can be equally aggressive as males, and might fight each other. If you have several females together, they won't pick on each other as much (at least, they shouldn't gang up on each other).
As for the squeezing into small spaces, you should try and minimize the risk of her getting stuck by blocking the hole in the pots. I read on here that bettas will try their best to get into any space, so you want to check each hole in each decoration by sticking your thumb through the hole; if your thumb fits, it's safe for your betta to swim through. If not, you should try and block it off.
Hope this helps!