The photo shows Housley, 18, with Mowry-Housley's son, Aden. He celebrated his sixth birthday Monday, and Housley was planning on attending the party.
"Happy 6th birthday Aden! Alaina was excited to come to your birthday party," Mowry-Housley wrote. "But as you said buddy, 'She lives in your heart now.' We love you."
The emotional post was met with several warm messages of condolences from Mowry-Housley's Instagram followers.
"Happy Birthday to you Aden you have a beautiful guardian angel watching over you," one fan wrote.
"Happy Birthday Aden, although I know your heart is saddened by the senseless loss of your beautiful cousin. You're right however, she'll always live in your heart. Praying for the entire family," another added.
"Sorry for your loss Lord be with this family," another wrote.
Earlier in the day, Mowry-Housley responded to a critic on Instagram who criticized her response to Housely's death.
"You are apart of the problem," Mowry-Housley wrote in a now-deleted post. "This is the prime example I spoke about in my interview. If you have love in your heart. You don't talk this way. You obviously missed the message. And don't you ever judge the death of my niece."
Housley, 18, was among the 12 victims at the Borderline bar shooting on Wednesday night. She is the niece of Mowry-Housley's husband, Adam Housley.
"Our hearts are broken. We just learned that our niece Alaina was one of the victims of last night's shooting at Borderline bar in Thousand Oaks," the couple said in a statement Thursday. "Alaina was an incredible young woman with so much life ahead of her and we are devastated that her life was cut short in this manner. We thank everyone for your prayers and ask for privacy at this time."
On CBS This Morning Monday, Housley's father, Arik Housley, said her funeral will be later this week.
"Alaina was beautiful. And she was my friend. Yes, there needs to be a change, but all the noise … nothing's been done," Mowry-Housley added on CBS This Morning.
The family said they believe Housley would have wanted the country to make sure this never happened again.
"She would want to happen what happened today, when we drove down that street, I saw people of all colors, all ages, people I've never seen, and I was born and raised in this town, hugging, holding hands, shaking hands," said Adam, a former Fox News journalist. "She'd want that to happen at the national level."0comments
"She wouldn't want this happening to anyone else," Mowry-Housley added.
Photo credit: Instagram/Tamera Mowry-Housley; Getty Images