The TLDR version:
Wanda was originally the wife of Maxi Gomez, a senior Argentine player at Sampdoria. When a young Mauro Icardi made the move from Barcelona to Sampdoria Gomez decided to be kind to the 18 year old and let him stay at his house to start with. Fastforward a bit and Mauro and Wanda begin an affair that breaks the Gomez's marriage. Mauro ends up stealing Maxi's wife and kids (he has Maxi's kids names on his arm). As a result Mauro is banned from the national team through order of Maxi's good friend Leo Messi.
Fast forward to now and Wanda is Mauro's agent and an "expert" on a Milan based football show. And she has spent the entire season throwing shit at the other Inter players and the Inter fans, which forces a wedge between Mauro and his teammates that eventually ends with him losing the Inter captaincy. And now the situation is so toxic that a move away from Inter has to happen.
Wanda is essentially a psycho agent and Mauro goes along with everything.
I mean, that's one version of the story.
The other version is that Maxi Gomez was having an affair with the housekeeper while Wanda was pregnant, and then was sued for sexual harassment. In the divorce, the court sided with Wanda and found Maxi as the one acting unfaithfully. Messi is Maxi's friend however and that group has made sure to slander Wanda / Mauro whenever possible.
Now, Icardi is insane in his own right and so is Wanda it appears. But Maxi (and via Messi) painting himself as the good boy victim in all this is madness.