WOW...That's some bull!!
What kind of family member "expect" you and your kids to pay or ultimately pay for a catering bill without notice? You invite people to an event or celebration with the assumption they'll be there to enjoy the moment with you and if they're willing to give you something in return, it's up to them. At least, that's my humble opinion. You plan ahead, and if you can't afford to pay for your own stuff, you just don't do it. Plain and simple. You don't call people (whether family or not) to ask them for money, donation or else. If I was you, I'll tell them to beat it....a***es!!!