Personally, I tend to draw a rather fuzzy line between real triangles and little plot complications. Like, if someone involved really can't make up their mind or has a real chance of changing it, that's a real love triangle, and I usually find them annoying (although there are some excellent exceptions). If it's just that someone shows up and causes a little trouble but it's really going to either have no effect or just push the main couple forward a bit through jealousy or whatever, that's not a real triangle it's just a little plot complication and whether I hate it depends how it's handled.
This looks like that second thing to me. Brother is there to force male lead to confront his feelings.