Based on your feedback I am pretty sure this is what you want - the trigger basically uses a nested query to pull back all the context records for any End Market that needs to be updated based on a new Context being added/updated/delete, sets the first one retrieved to active since that is the most recent date, and the others to inactive if they aren't already inactive...you may need to change the SOQL if it doesn't compile to use the correct relationship name...

trigger changeContexttoActive on Context__c(after insert, after update, after delete) {

if(!test.contextTriggerExecuting) { //check that the trigger isnt already executing
    test.contextTriggerExecuting = true;  //set trigger executed = true

Set<Id> emtoupdate = new Set<Id> ();

if (Trigger.isInsert || Trigger.isUpdate ) {
for (Context__c c: Trigger.New) {
emtoupdate.add(c.End_Markets__c);
}
}


if (Trigger.isDelete) {
for (Context__c c: Trigger.Old) {
emtoupdate.add(c.End_Markets__c);
}
}


List<Context__c> contoupdate = new List<Context__c>();

List<End_Markets__c> LEM = [Select Id, Name, (Select Id, End_Markets__c, Status__c, Effective_Date__c From Context__r order by Effective_Date__c desc) From End_Markets__c];

for (End_Markets__c em: LEM) {
if (em.Context__r.size() > 0) {
List<Context__c> ucontext = em.Context__r;

for (Integer i = 0; i < ucontext.size(); i++ )

if ( i == 0 ) {
        //only process if not currently active record
        if ( ucontext[i].Status__c != TRUE) {
        ucontext[i].Status__c = TRUE;
        contoupdate.add(ucontext[i]);
        
        }
}
else {
//only process if record is currently active, and needs to be inactivated
        if ( ucontext[i].Status__c == TRUE) {
        ucontext[i].Status__c = FALSE;
        contoupdate.add(ucontext[i]);
       }
}
}
} 

try {
Database.update(contoupdate); 
}
Catch (Exception Ex) {
system.debug(Ex);
}
 
}
} //end trigger