She knew exactly what she was doing. She'd probably planned it in advance and was really put out when your DP didn't go, as that made her plan less opaque, but she decided to chance her arm anyway. She was also relying on exploiting your Dad to make it too awkward for you to avoid being ripped off.
Even the most blatant of get-their-money's-worth bill-splitters don't usually try to split it unequally by the number of people. Their MO is to pig out as much as possible on the dearest options and then claim to have consumed 'an average amount' for one person. There's no way that even they could claim that three people should count as one for splitting purposes, as it would blow their own rules right out of the water.
Some people do exclude young children from the head-count for splitting, but that's done on the assumption that they'll be having a tiny plate of chicken nuggets and chips and a fruit shoot. If you order for them as if they were an adult (twice as expensive a dish as an actual adult had), they count as an adult for splitting purposes.
Of course, this all relies on a prior agreement that you will split, which, as far as I can tell, was only ever agreed for Dad's meal and not for everybody else's.
She left you without much choice on the evening, but you definitely need to challenge her about it now. Yes, it's an awkward situation - because SHE chose to deliberately make it awkward; none of the awkwardness was your doing.
It would still be CF behaviour if you had been the mega-rich sister and she thought she could blag a blatant sub from you, but to still pull this on you when your DP has missed out and you are clearly ordering cheap - both for obvious reasons - that is absolutely disgraceful.
Whilst not to be expected, I agree that she could easily have said "You know, you've done all the driving for us all and yours was only cheap anyway, plus we know that times are hard for you just now - we'll get this one, we insist."