My ex did it to feel alive and wanted, that was also one of the reasons i broke it off. She's obviously lying to you and the real question is how to deal with it. Have sex more frequently and you'll know if she cheated. Here's what i would do...
1.) Get better at sex with some tantra,keggle,jogging
2.) Have sex with her more frequently (3-4 times a week - let her crave for it)
3.) After 3-6 months the results will show.
If your wife had an affair she won't be interested in your brother in law anymore.
If she will then she's got some daddy issues to solve. In my case my ex's father worked 13h day and didn't see her very often, didn't compliment her and so on...google it
Good luck and don't give up!!