There is a general rule that one can be yozeh anyone with a bracha except berchos nehenim.
This concept doesn't appear to be very strong

We say somethings its permitted since there is a chiyuv - kidush friday night
We say somethings are permitted because it is a minhag, kiddish shabbat morning

If there is a brit milah on tisha bav, the mother drinks the wine, where does this fit in?

The rav says the blessing on the wine for the bicat erusin, the chasan and kallah drink it, where does this fit in. Presumably the blessing on the wine of kidushin come under the category of chiyuv.

Aryeh Shore

The Sugya is at the end of the 3rd Perek of R”H. The reason why you can’t be Moitzie Birchas Nehenin is because there is no Arvus for them. Even though if your going to eat you need to make a Bracha, but there is no obligation to eat. Don’t eat and don’t make a Bracha.

The Gemarah then has a Shaila about Birchas Nehenin by Mitzvos and they prove that you caould be Moitzie, like Hagafen for kiddush or Hamotzie for matzah. Since they’re obligations and you can’t do the obligations without a Bircha Nehenin, so you can be Moitzie.

The Bais Yosef dealt with Kiddush in the morning, which was aTakanas Rabanan, but even so he came out that it’s an obligation, so there is Arvus and you can make it for somebody.

The same holds by Bris, since it’s a Mitzvah.

The same by Eirusin, but some Rishonim hold that Birchas Eirusin is a Birchas hodah and not Birchas Mitzvah (See Rosh in 1st Perek of Kesuvos), so some M’sadar Kiddushin are To’am Biyadav (pour some on their hand and taste it, so not to be Pogem the Kos) so they’ll benefit from it themselves, so there shouldn’t be any Shaylos if they can be Moitzie others.

quote:

The same by Eirusin, but some Rishonim hold that Birchas Eirusin is a Birchas hodah and not Birchas Mitzvah (See Rosh in 1st Perek of Kesuvos), so some M’sadar Kiddushin are To’am Biyadav (pour some on their hand and taste it, so not to be Pogem the Kos) so they’ll benefit from it themselves, so there shouldn’t be any Shaylos if they can be Moitzie others.

Now that is interesting. I'll look for someone doing it. Thank you.

Aryeh Shore