The snarky answer is never. Never mention money because you should never work for friends and family. The snarky answer is also the best answer. Don’t work with friends and family. It causes more problems than it solves.
In the real world, it often can’t be avoided. You need to live up to obligations and expectations. In those circumstances treat it like any other business relationship. As uncomfortable as it may feel, act as professional as possible. At no point should it feel casual. Follow all the procedures you normally would, quotes, contracts, downpayment. Give them several opportunities to bow out gracefully in case they were looking for a freebie. There are worse things than criticism for being too serious and professional. If you feel obligated to give a discount or do a pro bono job certainly go ahead, but invoice like any other work.
Popular wisdom is that if you loan money to friends or family don’t expect to get it back. When it comes to working for friends and family, assume it will be harder, take more time and the work will be less appreciated than from a regular client.