Justin's mom, Pattie Mallette, posted the vid ... capturing the fam singing to Hailey as Justin holds a cake with candles. The second video shows how they did it up with a larger cake that Justin shoves into Hailey's face.

As Hailey blows out the candles, someone happily yells out "First married birthday!" They hug, and Justin says, "I just want to say, she's the love of my life, she's amazing..."

Hailey's cousin Kahlea Baldwin was also there.

Justin and Hailey secretly tied the knot in September at a New York City courthouse after taking out a marriage license. Still unclear why they wanted to keep it so under the radar.

They're expected to have a big ceremony sometime early next year ... possibly in Canada.

So far, so good.