I think Catherine should schedule a fun evening for herself that evening, to take her mind off things. Perhaps go to the cinema with a friend, and then spend the night away. If she plans to stay at home, she can read a good book, watch a film etc. Something she enjoys that will occupy her mind - not craft or hand work, unless she is watching a film a the same time. Listening to good, but soothing, music after she turns the light out might also help her not to fret while she's going to sleep, and, if Flavio and Juanita are in the same house, will muffle any noise they make.
Flavio and Juanita should make sure that Catherine does not feel in the way. Glancing at the clock as bedtime approaches is an absolute no-no! Flavio should make sure that he has taken care of all his household and child-care responsibilities before he goes to bed / leaves the house. He might even do a bit more than usual - especially if Catherine will have sole child-care responsibilities while he is with Juanita - as this will make it easier for her to let go, since she will feel considered and cared-for. He might leave a loving message on her pillow, for her to find when she goes to bed. Anything that will help her to feel more secure, valued and loved, really. If Flavio and Juanita will be in the house, unless the sound-proofing is excellent, they should make sure that they keep things fairly quiet.
There is also a responsibility on Catherine to make sure that Juanita doesn't feel in the way or unwelcome. J is likely to feel some uneasiness about starting a relationship with someone else's husband, so Catherine should go out of her way to reassure her by being welcoming, while being honest about the fact that she is experiencing some anxiety on her own account.