Just giving my opinion, but I took a girl to Frog and the Peach once and I thought it was just so-so. And for the prices you pay you should receive much better than so so. Of course, I only have been once so I am not an expert I am just giving my opinion from my years in New Brunswick. Another option is Catherine Lombardi and Stage Left.
My personal favorite though is Steakhouse 85. I don't know what type of diner you and your in-laws are, but Steakhouse85 is delicious and if I could choose anywhere to go to eat in New Brunswick regardless of money, 85 would be my first choice every time.