Erin and Sara Foster, whose dad is Kat's husband David Foster, showed their support for her latest swimsuit photo on Instagram on Dec. 29.
Things kicked off when David, 72, posted an image of Katharine rocking a black bikini and captioned it, "what baby," 10 months after she gave birth to son Rennie.
Some users criticized him, with one person saying, "Yeah, let's perpetuate the unhealthy narrative that women need to be back to or smaller than their pre-pregnancy size postpartum."
But Katharine, 37, felt differently and responded to the user directly. "Almost a year later after giving birth is an unhealthy narrative- go jump in a lake," she said.
The American Idol alum also created her own post to say that she had "zero" dieting and "zero" pressure to get in shape after giving birth. "Stop being so offended by what people post who have zero impact on your life and move on," the singer wrote this week. "Maybe you should have more of an attitude of like, let's say… 'oh that's nice he thinks his wife is hot.'"
Now, her stepdaughters are giving their take.
"If I can accept my step mom looking like this, you certainly should be able to," Erin, 39, wrote. "The photo wasn't altered or filtered and she's had no surgery. You can't shame someone for feeling cute after a baby whether she's sharing stretch marks or a six pack. Let her live."
Sara, 40, also pointed out a double standard for new moms. "If you [would have] had cellulite and stretch marks in the photo and the caption was exactly the same we would all be celebrating his post," she speculated. "But you don't and that's not allowed to be celebrated."
She then praised Katharine for her workout routine: "You have been working your ass off in the gym. Let's complain about women and husbands who post 3 week postpartum pics with their full body lipo, filters and altering apps that make us all feel like s--t," adding, "People are crazy." (David shares Erin and Sara with Rebecca Dyer, his wife from 1982 to 1986.)
Kat's Smash co-star Debra Messing also boosted her up, writing, "Brava! And send me your workout."
Katharine admitted she "found a great workout" and simply let her "body do its thing" after her pregnancy.
As she put it, "That's it. And guess what? I'll probably gain weight again at some point too. Who cares? BUT people freaking out about what my husband captioned seriously get a life."
Keep scrolling to take a look back at David and Katharine's relationship.