Relative Language

My brother in law and his wife have a bad habit of not showing up when they're supposed to and then of course showing up when they're unexpected.  I think this is very rude, last weekend my brother in law Mark and his wife said "maybe" we will stop over this Saturday, have dinner and play cards.  I assumed they were coming so I made dinner and appetizers expecting them to show up around six or seven, at eight o clock there was still no sign of them.  I learned that the next time someone says "maybe" well stop over I won't be so quick to prepare and plan the whole evening.  "Using relative terms without explaining them can lead to communication problems."  I only assumed they would show up because last time they said "maybe" they ended up coming.  I have now learned to get a clear definite answer because it can change my whole day of preparing and planning.  "Some relative terms are so common that we mistakenly assume they have clear meaning."  I can only blame myself because Mark never said for sure he and his wife were coming.  I know he felt bad so maybe next time he will be more clear also.