David Foster's daughters only have one problem with 36-year-old Katharine McPhee: that she’s a total stunner! "Our biggest issue with her is how hot she is. Her body is insane. When she comes over for family Sundays, I'm like, 'Do not get in a bikini,'” Erin Foster, 37, joked in a new interview with People. The girls also talked about how Katharine and David’s 34-year age difference has actually “played a role in a positive way.” Plus, their father recently opened up about his and Kat’s age gap, telling USA Today that it’s their “only problem” as a couple: “Everything else is in line, so it's not going to bring us down."