I'm not sure if this little tidbit of knowledge will be of any help to you or not. Upon doing research both in the US and in Scotland I found Agnes Airth born in the1300s married in abt. 1417 to Sir Edward Bruce. Her father was Sir William Airth of Airth. Every indication shows that family were those who created the village of Airth and later built the castle there. Agnes and Edward had at least two children - one the famous Robert The Bruce, and the second a daughter Mary Bruce who married a man by the name of David Hay. There are indications that in several places that Airth Castle was handed down to Robert the Bruce's son Alexander who lived during the 1400's.
I have not found any siblings for Agnes Airth up to this point- which may be the reason why the Bruce family stepped in line with the Airth Castle.
Documentation is sketchy at best when you get to history of this time frame in Scotland. Even church records, and land records are hard to take back this far. If you have found any good sources from the 1300-1400's please let me know.