Everyone should be treated equally. And parents should know how to treat their children equally. It’s their duty to not be biased. And if you think you can change this then you should first talk to your In-laws. Make them realize that your husband is their son too. They can’t just be partial between their two sons. No, doubt everyone has a soft corner for their firstborn. That doesn’t mean they’re allowed to treat the second son differently. You can make them realize their mistake. You can make them see all the efforts and hard work your husband has done for them. It’s their responsibility to treat them equally without any discrimination.