So I met some friends for lunch at the weekend and one of my good friends said that she knew that her BF's sisters BF (stick with me!) was going to propose in the coming weeks. (This is fairly common knowledge to everyone but the sister as her BF wants to have a party with both families the night of the engagement (assuming she says yes!) so everyone has been told to keep the evening free

Anyway, then another one of our group said something to the effect of "well thats you on the shelf for the next year to 18months!", as to her, you can't both get engaged at the same time as a sibling.
I can see why ideally the weddings/engagements would be one at a time, but sometimes if siblings are close in age, then these things happen? Once you don't plan the actual weddings to be too close together, then is there really an issue? But apparently this is a widespread convention?
My friend is now a bit paranoid that her relationship now can't move forward until his sister gets married, which could be ages! My friend and her BF are both early 30s and have been together about 2.5 years, so i can see why she wouldnt be too keen at the prospect of hanging around for an arbitrary amount of time just because of other peoples relationships.
Thoughts?