Thank you for your question. I believe I understand your dilemma.
No matter what, your sister will be looking for your support. Your parents are already going, and your absence will be interpreted as not supporting your sister. Attending the wedding will not signal approval; but rather signals support of your sister. I am sure you love her sister, and would want to be witness to all that happens to her as much as you can; so that later on, if she needs your help, you are better prepared to help her.
NOTE: just a side note: when they go for the green card, the legitimacy of the relationship will be questioned. They may even go to the extent of asking relatives, friends, neighbors and work associates about the legitimacy of the relationship. They have to attest that they are living together. So you will have plenty of opportunity later on to make official statements about your perceptions of the marriage.