I never think twice about dropping $100 for a good meal and I hardly ever buy clothes more than $100. But that's just me. DH laughs at me saying: other girls go for brand names but at least at the end of the day they have something to show for. All your money goes straight to your tummy!
Anyway, I think $70 is quite an expensive amount for an event like this without consulting everyone. So I think your sister is a little inconsiderate.
Before the economic downturn, my company used to have really generous travel allowances where breakfast was up to $35, lunch $45 and dinner $65 per head. It was great if there were a group of us were traveling together. My coworkers and I would go to the best restaurant in town and order the most expensive items (filet mignon, lobster ...) on the menu, with appetizers, red and white wine, desert samplers and all that jazz. We ate to the brim, gave a huge tip and never went over $65 per head.
So anyway, I have to say $70 is a quite a lot.