I remember your earlier post about these impending circumstances and asking what to do. I am so sorry about everything happening before the wedding, and so close at hand too. It has to be hard.
You have to do what you feel, fiance feels is right for you two. If you don't want to get married right after the funeral, postpone, people will understand. If you still want to go ahead, they will understand that, too, it might be a welcome relief/break in the saddness. These people who would be celebrating your wedding, are many of the same people greiving and dealing with the realities of the losses you and they have recently experienced, and the illnesses of your sister and future MIL.
Personally, I feel you/your fiance would regret not going to the funeral to say goodbye to his grandma, and that could be a source of future hurt/arguments, so I would go, and yes, it will definitely bring up feelings about your sister, and his mom's future. But you don't get a second chance at funerals, once they are over, they are over. Also, try to remember, the wedding itself is only a "formality", just one day out of a lifetime if love and committment - obviously, you have already committed yourself to your fiance, you two have been through lots together, postponed the wedding already, but you are still together. I know you want that day, you want the celebration of your love, everyone does, but the love will not change, whether the celebration is held now and tinged by sadness, or postponed till later when peopl may be a bit more happy. Personally I would probably go ahead and have the wedding. Unfortunately Life is full of ups and downs; at least if you go ahead and have it, his mom will still be able to be there, if you wait, who knows what else might happen, or how long it will end up being.
You have to do what you two feel will be best.
My husband's grandma died unexpectedly a couple weeks before our wedding, too, although that is more time than you have, it was till a blow. She lived halfway across the country (a 15 hour car ride), but we still made the trip out there by car to go to the funeral; I knew he would regret it if he didn't go, and I wanted to be supportive, too, she was a very loving sweet lady. The trip itself was stressful, and it put a further strain on finances, since I had lost my job to downsizing a few months before that. We had to cancel our honeymoon (we had been planning a big trip to Florida, but ended up doing a FREE weekend thing at my family's lake cabin in WI),and we did the wedding very inexpensively. We still had a wonderful day, though we missed his grandma's presence. People still talk about our wedding day and how nice and relaxed it was, even though we didn't spend much money on it. Our Ninth anniversary was last Friday, we have gone through lots of family drama, 2 4 month long lay-offs for me, 3 job changed for DH, we have moved 2 times, we have had two children by c-section, and lost one through miscarriage, and lost DH's step-dad and a close cousin and friend this year. Life happens, but if your love and committment is strong, you can get through it together and come out even stronger on the other end.
Good Luck deciding, you have to do what you feel is right, not just try to please everyone else.
Jessie