runners-up vs side-ins

[Eugene2114]

Hi,
Is there a certain rule regulating the plurals in such cases as: side-ins, add-ons, throw-ins, but runners-up?
Regards.

[Beeesneees]

The obvious difference is that runners-up are people...
In the same way as brothers-in-law, sisters-in-law and mothers-in-law.

I don't know whether that's the rule or not though.
_________________
Cheers m' dears!

[Eugene2114]

Thanks, Beees. If that is the case, and " the golden rule is that..." one should memorize those plurals.
Regards.