Stewart-991 Isabella married to Douglas Maitland Bruce & Edmonstone
Isabel b 1350 d Robert s of James s of John s of Alexander
m Robert de Bruce s of Thomas de Bruce
thepeerage.com only lists two husbands for Isabella d of King Robert Stewart: Douglas & Edmonstone.
A different Isabel would explain this discrepancy.
Alison Weir is the source on peerage
geneagraphe does not provide sources and I have limited experience with this site so I couldn't comment on how dependable it is.
FMG only lists Douglas as a husband.