12:14 PM PT -- Beyonce's mom, Tina Knowles, just liked Mathew's post on IG, making her the 2nd family member to publicly acknowledge the birth.
Beyonce's dad weighed in on the birth of his twin grandkids five days after the infants weighed in.
Mathew Knowles took to social media Sunday with a digital birthday card to Beyonce's new baby boy and girl with a tag that said ... "They're here!" The card read ..."Happy Birthday to the twins! Love, Granddad." He also threw in hashtags like #beyonce #jayz #twins #birthday and #happybirthday.
TMZ broke the story ... the twins were born Tuesday but have remained in the hospital because of a "minor issue." Beyonce has also remained in the hospital with her newborns.
Mathew is the first family member to publicly shout-out the birth.