you would need to be careful brazing, in some steels the heat from the brazing will cause weakening through over tempering, although I believe the reynaulds 531 is one of those manganise-molysomething-or-other alloys and as such it won't have been heat treated, so could probably handle brazing much better.
is
@Reynard a metalaugurist ? she might have something interesting to say on the matter
Failure of composites is where I'm at, but I've a reasonable all-round grounding in Strength of Materials. Am somewhat out of practice though, which basically means I'd have to find my old notes and texts in order to do the maths these days...
TBH, me personally, I'd want a sleeve over that at the very minimum. Dunno what's gone on there, but I wouldn't trust a lesser repair. If it's only a bike for the odd occasional ride, then a sleeve would probably be fine, but if it's a bike that will get a good bit of use, it may well be a better option to get the seat stay replaced. You're dealing with cyclic loading on the stays as your weight rock backwards and forwards while pedalling (as opposed to a static load), and as it is, it will eventually fail - you've three stress raisers AKA holes grouped close together, and cracks will propagate quickly and catastrophically between them. Especially on what is a relatively thin section of hollow metal.
I'm not au fait with brazing as such, but plugging the holes with a different metal or alloy (which will have different physical properties to steel) can also cause issues eventually, as will the heat involved during the brazing process.
@Illaveago had a similar issue with a hole in a chainstay, and got that replaced.
