First, know that you do NOT have to explain anything you do to anyone (unless it affects them directly). The best thing I've found to say in a situation like this, where you have to be careful not to be offensive, but to set your boundaries, is "I'm sorry you feel that way." That puts the onus back on them, because the truth is that they've chosen to feel that way. It took me years to learn not to accept responsibility for the way everyone else felt. The first temptation is to let 'er rip and say exactly what you think, but in a family situation, where you don't want to break bonds, it's not so easy.
Secondl.. no one has any right whatsoever to opine about your financial situation... whether they approve of how you spend your money or not. Period. The problem with folks like that is that they have no boundaries, so they don't respect them for others. Good luck!