Hi Tiffany! Technically, since the shower is your time to receive gifts, you aren't really required to give back anything. Your sister and cousin recognize your value to them and want you to sit back and allow them to demonstrate it. However, if you feel prompted to buy something, I think a bouquet of flowers, a box of chocolate, or a nicely written card would suffice as a great sign of appreciation and gratitude!
It isn't about being required to do anything. It's about being polite and courteous. It is traditional and polite to take a hostess gift whenever someone is hosting you- especially a gift giving event.
Try to personalize the gift as you would for any other gift. Do you know their favorite wine, a scent they love for a candle, a luxurious sweet or dessert they probably wouldn't buy themselves? A bouquet of flowers is always discouraged as a hostess gift unless it is already arranged in a vase, as the last thing a hostess needs is to have to take time to arrange flowers when she is busy greeting guests.
I have given hostess gifts for all of my parties and showers. It doesn't need to be expensive, just thoughtful and nicely wrapped. I gave my bridal tea hostesses nice pictures frames, and my kitchen shower hostesses little bowls from a well-known local potter. I also like the idea of a plant in a pretty container...maybe an orchid or hydrangea.