Abetterday, I do understand how you feel. One just doesn't end a marriage as if it had never happened. You mentioned that you only divorced for 9 weeks, I'm sure you're still having lots of emotions attached. If you feel like going, by all mean, you should. Even if you're no longer considered part of the family, they have no rights to prevent you attending as a friend. Please also remember that none of you (the ex-wives) is officially his widow. Socially speaking, you are all having the same status. If they are attending, why not you. If you run into them, greet them and say something nice. If they act improperly, just smile and simply turn away. Treat them as one of the guest and you'll be quite alright. However, I still think you should keep a low profile under the circumstances. Don't arrive too early to shorten interaction with his family. Take a middle or back seat to avoid attention. Greet everyone you knew gracefully. Approach anyone you feel like approaching. Just act normal, be subtle and try your best to stay out of confrontation with anyone. If you are just trying to show you respect, there is nothing to be ashamed of. If you are attending the interment, try taking your own car to prevent yourself being stuck at the backseat with a bunch of strangers. If you have children (with him), stay close to them to offer your supports. When it is over, greet the family and leave. Other than that, wear something simple (in black or grey) and put on your most comfortable pair of pumps. That's all you need to do. I hope it helps.
BTW, my ex-husband also recently passed (predeceased by my late husband last August). I handled both of their arrangements - for the sake of my two daughters.