Is there a way to do this tactfully?

I would suggest she tell her sister she thought about it and would like to make her own speech. Let the sister make her lousy generic speech and then have your wife say what she wants to say. I think that’s the cleanest.

Wife didn’t want to write her share of the speech.
I think since she left it to sis, that is what they should go with. If she wanted to critique, she should have done her share.

“Let me add or tweak this speech a bit.”

Or another tactic would be for her to say that’s a good first draft but I’d like to include some more personal anecdotes about them and then edit them in. (What @chyna was saying at the same time.)

The people to think of first in this matter are not your wife, not her sister, and not the other guests, but the parents. What will make for the most enjoyable and meaningful experience for them?

It doesn’t sound like the answer is the speech that Sis has in mind.

Since Sis has presented a first draft, apparently expecting comments, it’s your wife’s chance to say, “Thanks for giving me a chance to collaborate on this. How about if we make it a little more personal?”—and then return a suggested revision.

Some people are really grateful for suggestions. They’re not happy with what they’ve written, but they don’t know how to fix it. If your wife takes a collaborative approach and not an antagonistic one, maybe they can work together to create something that pleases both of them—and that will please the parents.

Failing that, your wife could say, “Ok, why don’t you go ahead with your remarks, then, and I’ll write a little speech of my own?” And then she’ll have to do it. As she could have done in the first place.

I can’t help thinking, though, of my cousin’s wedding. She’d chosen her younger brother to emcee at a very large reception, and he got up and told a string of jokes that I thought were tasteless and even insulting toward the bride. Another cousin, their sister, was at my table, and when she noticed my probably shocked expression, she said, “I imagine a lot of people are a bit upset and thinking my sister is going to be bothered by this—but believe me, my sister knows our brother very well and is not surprised by anything he’s saying.”

The parents of your wife and Sis know Sis very well. Ask your wife if it’s likely to bother them if Sis makes a stupid speech. For the sake of making it a great day for the parents, I hope there’s no need to regress to old rivalries, which is what this sounds like. It’s not your wife’s celebration, it’s theirs. Smiling and making the best of it might be the kindest thing she can do.

I think your wife tells your sister she’s been thinking about it, and wants to be able to say a few thoughts of her own that night, and ask her sister if she minds. The sister will say she doesn’t mind, and then your wife can say, “ok great, I’ll send you what I’m thinking so you can look at it once I write something up.” Don’t be prepared with the paper, let the sister think she has some control.

Now, she has the sister’s agreement, and the sister can read her own thing, and your wife can add her own thing.

Once your wife sends over her paragraph to her sister, maybe the sister will edit her own after seeing the idea behind it to make it more personal. If she doesn’t edit her own, she doesn’t.

If the sister doesn’t agree to let your wife say a few of her own words when your wife asks, then your SIL is unreasonable, and your wife should do what she wants anyway.

Maybe tell your sister that others want to give there speech as well, so to
make it happen everyone has only about two entrances/ or paragraphs to
give and that will
uplift the bide and groom with positive vibes ,as well for the whole group.
Another idea is to have a theme for each speaker.
For example: early years when the couple met, careers etc.
Each speaker only touches on one particular event in the couples life.
If possible have those that will be speaking draw from a hat the topics or
theme
that they are to work on.

Thanks all. My wife was initially very upset about this. She hates confrontation and doesn’t want to hurt her sister. She has had some time to think about it and she’s just going to add to it and not make a big deal about it.

Add to it sounds fine.

Update: Sister got all butt-hurt about her really bad joke, but my wife asked if she would like it said about her and her husband and I think she is, maybe, deciding against it. We’ll see.

One question to ask Sis is: who will be hurt or offended if she says it? and who will be hurt or offended if she doesn’t?

It’s a safe bet that no one would come up to her later and say, “I enjoyed your speech, but it would have been much better if you’d told a really crude joke about your parents.”