Hi Jainam,
I used your logic and I still got the same 50% code coverrage.
Below is the code and the class, the uncovered code is highlighted:

Apex code with block separator to highlight the uncovered code:

trigger SumTotalMonths on TargetX_SRMb__Family_Relationship__c (after insert,after update,after delete,after undelete) 
{
Map<ID,TargetX_SRMb__Application__c> applicationMap = new Map<ID,TargetX_SRMb__Application__c>();
    List<ID> appID = new List<ID>();
    
    //check for the DML operation, for Insert,update and undelete
    if(Trigger.IsInsert || Trigger.IsUpdate || Trigger.IsUndelete){
        for(TargetX_SRMb__Family_Relationship__c Rel1 : Trigger.new) {
            
            //Add all new  AccountID in the list
            if(Rel1.TargetX_SRMb__Application__c != NULL && Rel1.Total_Months_in_this_Position__c != NULL)
                appID.add(Rel1.TargetX_SRMb__Application__c);
        }
    }
     // check for DML operation for update and delete
    if(Trigger.IsDelete) {
        for(TargetX_SRMb__Family_Relationship__c Rel2 : Trigger.old)
        {
            if(Rel2.TargetX_SRMb__Application__c != NULL && Rel2.Total_Months_in_this_Position__c != NULL)
              appID.add(Rel2.TargetX_SRMb__Application__c);
      }
    }  
   // check if application list is not empty

    if(appID != NULL && appID.size()>0){
        // Add all the applications in the map to map IDs with total months employed

        for(ID ApplicationID : appID) {
           applicationMap.put(ApplicationID, 
          new TargetX_SRMb__Application__c(ID=ApplicationID,Total_Months_Employed__c= 0) ) ;
      } }
   // Calculate the total months employed based on value in the total months in this position
    for(TargetX_SRMb__Family_Relationship__c Rel : [ SELECT ID,TargetX_SRMb__Application__c , Total_Months_in_this_Position__c
                                                   FROM  TargetX_SRMb__Family_Relationship__c WHERE TargetX_SRMb__Application__c  IN :appID ]) 
     
     {applicationMap.get(Rel.TargetX_SRMb__Application__c).Total_Months_Employed__c += Rel.Total_Months_in_this_Position__c;
                          }
    
    //commit to the database
    Database.update(applicationMap.values());
}

Here is the test class:


public class GetTotalMonthsWorked {
    static testMethod void CalcMonths(){
        
 //create new Account
Account acc = new Account(Name= 'Unassigned Contacts', Industry = 'Education'); 
 insert acc;
     
// Create new contact
Contact c = new Contact(FirstName= 'Sophie',LastName= 'Test', Accountid = acc.id, Gender__c  = 'F', Email= 'Testset@test.com');
 insert c;
// Create new application
TargetX_SRMb__Application__c app = new TargetX_SRMb__Application__c(TargetX_SRMb__Contact__c = c.id,TargetX_SRMb__Stage__c = 'In Progress',
                                                                      TargetX_SRMb__Status__c= 'Incomplete');
insert app;
        
// Create list of relationships
List<TargetX_SRMb__Family_Relationship__c> RelList = new List <TargetX_SRMb__Family_Relationship__c>();
        
for( integer i=0; i<200; i++)
{
            
 RelList.add(new TargetX_SRMb__Family_Relationship__c (Name = 'TestRel' + i, TargetX_SRMb__Contact__c = c.id,
 End_Date__c = date.ValueOf('2018-03-02'), Start_Date__c = date.ValueOf('2016-03-02'))); 
            
        }       
insert RelList;
//confirm if the total months worked is updated.
TargetX_SRMb__Application__c App1 = [SELECT ID,Total_Months_Employed__c FROM TargetX_SRMb__Application__c
                                     WHERE id = :app.id Limit 1];
System.assertNotEquals(1000,App1.Total_Months_Employed__c );
System.assertEquals(500,App1.Total_Months_Employed__c );
        
//Delete relationship record from application
 TargetX_SRMb__Family_Relationship__c deletedRel = [SELECT ID, Name,TargetX_SRMb__Application__c 
                                                    FROM TargetX_SRMb__Family_Relationship__c 
                                                    WHERE TargetX_SRMb__Application__c = :App1.Name LIMIT 1];
delete deletedRel;
                                                        
TargetX_SRMb__Application__c App2 = [SELECT ID,Total_Months_Employed__c FROM TargetX_SRMb__Application__c
                                            WHERE id = :app.id Limit 1];
System.assertNotEquals(1000,App2.Total_Months_Employed__c );
System.assertEquals(500,App2.Total_Months_Employed__c );
       
             
   }

}


Thanks for your help!
Mariam