How do I make a field mandatory if the previous field is set to yes?

I need some help with JavaScript code, please. I have a form that asks a question, if you answer yes, then more fields appear. I want to set it to if yes, those fields become mandatory.

This part works fine:

  <label for="export">Do you export?</label>
  <ui:inputSelect value="{!v.account.Export__c }" aura:id="export">
    <ui:inputSelectOption text="No" />
    <ui:inputSelectOption text="Yes" />
  </ui:inputSelect>

  <aura:if isTrue="{!v.account.Export__c == 'Yes'}">

I would like to make the below mandatory only if yes is selected:

    <label for="export-where">Where do you export?</label>
      <ui:inputSelect multiple="true" aura:id="export-where" value=" !v.account.Export_to_where__c}" change="{!c.handleExports}">
        <ui:inputSelectOption text="Option 1" />
        <ui:inputSelectOption text="Option 2" />
        <ui:inputSelectOption text="Option 3" />
       </ui:inputSelect>

      <label for="export-what">What do you export?</label>
        <ui:inputTextArea aura:id="export-what" value="{!v.account.Export_what__c}" maxlength="32768" />                          
    </aura:if>

The below JavaScript works as all being mandatory.

else if(getCurrentStep == 3){
    this.removeBorder(component, 'export');
    if((account.Export__c == 'No')) {
        return true;
    }
else{
    component.set("v.errorMessage","* Please fill all the required details");
   if(account.Export__c =='Yes' || account.Export_to_where__c == 'undefined' || account.Export_to_where__c == null || account.Export_to_where__c.trim() == '') {    
        this.addBorder(component, 'export-where');
    }
}

How do I change it so that when the Export_to_where__c question is filled it validates? Any help would be appreciated.