" my little sister just got a male betta we put her in one of my empty 2 gallon tanks"
Why would you stuff your little sister in a 2 gal tank? sorry could not resist.......
I have been fooling with and raising bettas now for over 25 years and have some males as well as females over 5 years of age. I have found that simple filtration is certainly better than no filtration and pico sized tanks are ideal, contrary to what is written on them being perfectly happy in a brandy snifter or mason jar.......I also keep mine at 70 to 72 deg or at the bottom of the temp range normally given for tropical FW fish, and have even kept them as low as 68 deg, and have never had a problem. Most of my older males are in 68 to 70 deg water and do just fine....They do not like overly bright tanks and prefer to have a plant to rest on.
All we feed is 4 to 6 pellets a day at most to each betta......
I think its was a great decision on your part to house it in a 2 gal tank.......Its hard to say how ong the one you bought will live as there is no way to tell the age. A lot of males sold in stores are grown males that were originally used for breeding, so they may already be 2 or 3 or more years of age when you buy it....The ones we have that I know how old they are are ones we raised up, so they do have a fairly long lifespan if fed and housed properly....I would have to venture a guess as 5 or 6 years of age as being about it for a well kept betta.