To answer your question, we'll discuss who the characters are in relation to Rebecca.
Giles Lacy: Giles is Beatrice's husband. Beatrice, meanwhile, is Maxim de Winter's sister.
In the novel, Maxim tells the narrator (his wife) that Rebecca had indeed made a play for Giles when she was alive. Accordingly, Maxim had invited Giles and Rebecca to spend a weekend at Manderley. During their time together, Giles went sailing with Rebecca. When the two returned, Giles showed all the outward physical signs of having engaged in improprieties with Rebecca. His manner was more jovial than usual, and he laughed and talked with uncharacteristic enthusiasm.
Maxim also noticed a familiar look in Rebecca's eye, one that was always conspicuous when she had made a conquest. Although the text doesn't explicitly confirm that Rebecca slept with Giles, we can infer that she did (based on her and Giles's behavior upon their return).
As for Beatrice, Maxim reveals that she had never liked Rebecca. Despite this, the brother and sister didn't discuss their feelings about Rebecca. So, was Beatrice upset that Giles may have slept with Rebecca? The text implies that Beatrice suspected her husband's infidelity and was upset. However, she chose not to make her feelings known to Maxim. After all, Beatrice had no proof that her suspicions were true. Remember that Giles went sailing with Rebecca alone.
Frank Crawley: Frank is the overseer at Manderley. The text tells us that Rebecca made sexual overtures to Frank when she was alive. For his part, Frank did everything he could to distract her. When he could no longer bear it, he approached Maxim de Winter to tell him that he needed another job away from Manderley.
At the time, Maxim had not understood Frank's seemingly resolute desire to leave. He was under the impression that Frank had been happy in his job. The two spent two hours arguing about Frank's decision. Eventually, Frank confesses his true reasons for wanting to leave: Rebecca pursued him persistently. She (Rebecca) went to Frank's home many times to proposition him. According to Maxim, Frank never gave in to Rebecca's demands.
Jack Favell: Jack is Rebecca's cousin. The text is clear that Rebecca slept with Jack. In a conversation with the narrator, Maxim reveals that Rebecca often spent nights with Jack at a private cottage on the grounds of Manderley.