Could Someone else fill me in on what Google isn't.
Some people will tell you they all need to be the same size, age, or sibblings but in my experience size, age and being sisters or not has nothing to do with how well they will or will not get along. I have had giants in with very young females - even a male (which I, personally have had good luck with but I really do NOT recommend it). It all comes down to each individual bettas personality and I think howdecorated/planted the tank is. There are some females that just can not live with other females and you need to be prepared for that to happen - you may have to remove the aggressive female and you may also end up with a badly injured or dead female. They may be fine one day and the next all hell breaks loose and you have several injured (which is why I keep a very close watch on my male.)
Speaking of males, another thing you need to look out for is petco had a tendency to mislabel young VT males or male PK as young VT females - thats how I ended up with a male in my soriety. I picked up 4 very young VT females from petco one day and put them into the 20G- they all spouted like weeds but the one, he kept growing and growing...right into a VT male.
I just switched over from plastic to live plants so my tank is a bit sparse at the moment but here is what it looks like - so you get an idea:
its still a little to bare for my taste but I'm sure once the plants grow in a bit - it will look lovely :) i still have some fake plants to provide cover at the surface