So my grandmas sister had twins, and my grandma says twins skip a generation, plus I have a BMI over 40, could I have twins?

Possibly. I am not an expert on twins but i can say that with a bmi of 40 you have to be careful. Yes you can get pregnant but it is usually harder to get pregnant and have a healthy pregnancy at a bmi >40. In addition the baby is at a much higher rate of diabetes as he/she grows up. You are very young, please try to get healthier don't worry about being skinny. Good luck.