My sister (mother of the bride) recently passed away. I would love to say a few words to my beautiful neice - but I don't want to turn make everyone cry. Could you suggest a few words to say?
Since the loss is so new to everyone, it's not necessary to make it a big part in your speech.
Toast to your niece as you would have done anyway, gushing about how beautiful she looks and how much she has touched your life, what a wonderful wife she will make, and your hopes for her future.
To mention her mother, you might open your speech with:
"Thank you for letting me take a moment to toast my beautiful neice. ________ you look absolutely radiant today, and very much in love. As you walked down the isle I'm sure I saw an angel on your shoulder - your mom wouldn't have missed today."