My fiancee's family is overweight. Should I be concerned?

If you're reconsidering marrying the woman you love out of fear that she'll gain weight, you don't really love her. I understand your concerns, so maybe you two should start exercising together and living a healthy lifestyle NOW to try to minimize the possibility of EITHER OF YOU gaining weight in the future. I don't know how old you are, but life happens and over time people start to age and maybe gain weight, especially after a pregnancy. Some women's bodies never go back to what they were. If you think your body or your fiancee's body will always be the same as it is now, dream on. And if you think you can't deal with that in the future, maybe you should do HER a favor and back out now.