To be fair, he also didn't acknowledge Melania's birthday on social media. 

President Donald Trump spent Sunday tweeting about many things, including the birthday of the U.S. Navy — but did not shout out youngest daughter Tiffany Trump, who turned 26 on Oct. 13.

To be fair, Trump doesn't seem to be one for social media birthday and anniversary posts for family members, since he didn't wish wife Melania Trump a happy birthday on Twitter, either. But the lack of a message was made even more glaring when other members of the Trump family posted well-wishes for Tiffany on social media.

Sister Ivanka Trump posted an old photo of the two of them on Instagram, writing, "Happy, happy birthday Tiffany! You have grown into an amazing woman, despite my many babysitting fails!"

Her sister-in-law Lara Trump also posted a birthday message, as did Tiffany's mom, Marla Maples.

Unlike her siblings, Tiffany has played a smaller role in her dad's presidential campaigns and administration. According to a Politico report from earlier this year, she is "not expected to play any role in the [2020] campaign, apart from appearing at the occasional event."

"Members of the Trump family are valued advisors and integral parts of the campaign," a Trump 2020 campaign representative told People after the report went up. "Their input and involvement were key to victory in 2016 and will be so again in 2020."

Donald Trump & Tiffany Trump
Credit: Chip Somodevilla/Getty Images

Tiffany, who was raised by her mother in California, has reportedly had a strained relationship with her father.

"Since the inauguration, Tiffany and her father have sometimes gone for months without speaking, and she went a very long time without seeing him," a source previously told People. "The last time she was at a family function with him, it was awkward for her and she didn't feel totally welcome."

Earlier this year, the president's assistant Madeleine Westerhout was reportedly let go after she spoke indiscreetly about Tiffany. According to Politico, she said said she felt she had a stronger relationship with him than Tiffany or Ivanka did, and that the president didn't like being photographed with Tiffany because he thought she was overweight.

Trump later denied the claim, telling reporters, “She’s a wonderful person and she studies so hard. She’s a great student. She’s a great person... Tiffany is great. I love Tiffany.”