Hi Colin,
You are best to talk to the airline (AC) directly, rather than asking the readers of PPRuNe. There are literally a dozen reasons why that may have been done, none of which are personal.
It is possible those passengers were airline or codeshare staff (deadheading crew); it is possible those two passengers were known about ahead of time while your sister was not...you assume the Captain knew about your sister...
There are a lot of considerations to take into account and it is possible that it was only the Captain relaying the information from another location and this was not the Captains' decision.
Unfortunately, the tariffs are very clear when it comes to closing off a flight and if you're a minute late, you may as well have arrived after the airplane left. The cold hard truth is they are under no obligation at that point to reopen the flight. When they do so it is normally only for operational reasons and where they are confident doing so would not unduly impact the departure of the aircraft. For example, maybe waiting the extra 10 minutes for your sister would have exceeded the crews duty time at the arrival airport - in that case, they simply can't wait - even though it is one person, they will end up delaying hundreds. Now instead of telling one passenger they are not going, the Captain has to tell hundreds of people and a number of crew members. In another example, those two gentlemen may have been able to get through security within the existing regulatory timeline; your sister may not have been able to do that.
It's just not a black and white decision, which is why you're best to go directly to the airline...except, and I hate to be so very blunt because I do empathize, but they're not at fault.