Your BIL has really dropped the ball. If he couldn't afford to buy two tickets, he should have scrapped the idea and got her another preset instead. Only getting one ticket means a big imposition on someone else.
Also, while some people are ok with going to concerts by themselves, I'd estimate that the majority of people would prefer to go with a friend.
And if money is a bit tight for your sister and BIL, it was foolish to buy her a gift that would result in her having to fork out more for travel and accommodation, etc.