Matron of honour speech

Hmm, do you mean that during your speech you would ask them? I don't know if that is the best idea, despite your good intentions. If you mean that they would already have accepted, then a simple nod to their future title would be ok without going into detail.

I would ask separate from the speach.

I'd be pretty upset if I was in their shoes and that was asked at my wedding. It takes the focus off of them and their new marriage.
The day is about them, so bringing your child into it seems a little inappropriate. I do agree with the PP, if you ask beforehand and they say yes, then adding something into the speech about how youre so glad your child has such wonderful godparents would be fine. But asking them during the speech seems super inappropriate.
But I don't know them! Maybe they'd be chill about it, like my one friend who OK-ed my other friend proposing at her wedding. But for me, and I think most brides, I don't think the wedding day is the right time to ask them.

I would skip mentioning it during the speech as the speech is supposed to be focused on their love and marriage, not you having a child.

I would probably avoid this. I don't think you have any bad intent at all, but I would probably keep the speech focused around celebrating them getting married, and not celebrating them being your child's godparents. You could ask them a week or two after the wedding, which would give them a new reason to celebrate now that their wedding has passed!

I agree with everyone else that this isn't appropriate to include in your speech at their wedding. And even if you still think it's ok (I realize, of course, that you know your sister best), ask yourself what are the potential pros and cons with doing this. There's really no benefit to the happy couple for you to ask in that way, but there is a definite possibility that they won't like it.

I don't think it is appropriate to ask at a wedding for a couple of reasons. Number one being that this wedding is about them so by asking them to be godparents that takes the attention away from them and makes it about your baby. Number two is if they would for some reason not want to be godparents they might feel pressured to say yes because you asked in front of a bunch of people.

Not that they would say No, but asking them publicly in front of such a big audience makes it impossible for them to have the option to decline. You'd really be putting them on the spot. Personally, I would probably appreciate a much more intimate request at a separate time so I can soak in that exciting moment separately and give it the attention it deserves outside of a wedding ceremony.

As everyone else has said, not a good idea to do this. Keep the focus on them and their wedding vs. bringing your baby into things. They may be okay with it, but they may not. If it were me, I wouldn't be happy and it's because I already feel like most of our time with family goes to the kids now because they're all little, so I want to have my one special day for me and my fiance.

My fiance's sister asked us to be her godparents and I really enjoyed the way she did it! She incorporated it into our Christmas gifts for the year, she got me a picture frame with me holding her baby and it has a little poem about a godmother and she got my fiance a t-shirt with a video game controller on it that says "upgraded to godfather." It was really sweet of her to do and made us feel very special.

Agree that it's not a good idea to ask in the MOH speech

Definitely ask them privately and not on their wedding day.