This is such a hard situation. But as hard as it is for you and your DH I think you need to respect the bride's wishes or face the fact that you may cause pain on her wedding day.
Loosing a parent suddenly and at such an early stage in you're life is really hard. My mother died suddenly when I was 19 and I can tell you from personal experience that it was hard to see other girls with their mothers. For years I couldn't stand Mother's Day. And had a real issue when I heard daughters arguing with their mothers. There is a certain horrible stage of hurt and jealousy that a person has to go through. Sometimes it can take years.
20 years later and it still stung a little getting married without my mother being there. Luckly I had my dad.
Going through something similar, I can see how it would add salt to the wound. It really stinks that you and DH have been asked to stand aside on such a wonderful occation but please know that the bride is not doing it out of selfishness. She is still mourning and as hard as it may be out of respect for her loss I think you need to abide to the wish. In the long run, the bride may see it as a compassionate wedding gift.
OK, I just felt I needed to add something. I just read some of the posts and found some of them down right rude. I don't mean to at like the snob and tell anyone they have not right to post on this because they don't belong to the "lost parent" club.... but.
First of all it's only been 2 years and people heal differently and respond to a tragity like this differently. Sure, some of you know someonoe who went through this and bounced back quickly, but that doesn't mean she can or has. You don't know what she has gone through unless you have gone through it yourself.
Have some compassion ladies. She will not have her father to walk her down the aisle. She will not have her mother tend to her dress and veil. How would that make YOU feel? Think hard about it..... "step-in parents" or not, it won't be the same.
Strong character or not....She will have sadness on her wedding day.
If you must, yes, talk to her about it. But please!!!!!! Don't pressure her into giving in. If she's not ready, she's not ready.