Baby mama drama....it's getting old.

My husband has a child from a previous relationship. 
I love my step son like my own. 
The problem here is that his mom is ALWAYS trying to be fresh with my husband and always trying to get back together with him. EVEN WHEN SHE IS IN A RELATIONSHIP! 
She's always asking mutual friends if my husband is happy and how we are doing. And always tries to flirt with him through text(he ignores her texts unless it has to do with my step son) 
It's getting so old though. We have been together for 3 freaking years....and she has been with her boyfriend for 2 years and has two kids with him!! I just don't get why she keeps this up. Please tell me things get better cause I'm about to flip..again.