The women honored one another with some sweet gestures on the special holiday.
Haack shares two children with her ex-husband, Tarek El Moussa, and received a special bouquet of white flowers from Young, who is engaged to El Moussa and will soon be the stepmom to the former couple's two children -- daughter Taylor, 10, and son Brayden, 5.
"Happy Mother’s Day to the best mom ever!" Young expressed in the card sent with the flowers. “We love you so much!"
Haack took to her Instagram story to share a snapshot of the sweet gift, writing, "Thank you @heatherraeyoung" along with a smiley face and heart emoji.
Haack -- who is also the mother of 1-year-old son Hudson with ex-husband Ant Anstead -- reciprocated the gesture, sending Young a bouquet of colorful blooms, sent from herself, Brayden and Taylor.
Like Haack, Young also posted a snapshot of the considerate gift to her Instagram story, adding, "Thank you @christinahaack happy Mother’s Day 💕."
Young also addressed her role as a stepmother in her relationship with El Moussa, and celebrated feeling included among those being recognized during this holiday for the first time in a touching Instagram slideshow post.
"Happy Mother’s Day to my beautiful Mom and to all the amazing Mother’s in my life: my sister, Angelique, my Mother-in-law, and today is also the first time I feel like I get to celebrate myself a little today, too, as a bonus mama," Young captioned a slideshow of photos of herself and several other important women in her life. "Over the past couple of years of being a bonus mom, I’ve had the most special experience of being in Tay and Bray’s lives, watching them grow up, putting them first, being there for them no matter what, and bonding with them on a whole new level. It’s the biggest blessing in my life and even though I never knew I wanted or needed it, now that I have two amazing kids in my life I can’t imagine my life without them."
El Moussa sweetly commented on his fiancée's post, sharing, "We are so lucky and couldn’t have asked for a better step mom. The babies adore you bunny love."
Last month, El Moussa and Young opened up to ET about their relationship and co-parenting. El Moussa and his ex-wife still star opposite one another on their HGTV show Flip or Flop, and he said that the friendship between himself and Haack couldn't be better.
"[Our relationship] is in a completely different place," El Moussa says. "She and I are getting along, we're co-parenting, getting along together, and I really think the episodes are getting better and better."
For Young, she says, "It was never an issue" having her fiancé work with his ex-wife. "Because it's work. It's his job," she explains. "They've been filming for 10 years, who am I to dictate? I am very supportive. We support each other through everything."
For more on Young, El Moussa and Haack's blended family dynamic, see the video below.