Apex Sum on Opportunities

trigger UpdateToTalFeeOnOpportunity on Matter__c(after insert,after update,after delete){
	Set<Id> oppIds = new Set<Id>();
	List<Opportunity> lstOppToBeUpdated = new List<Opportunity>();
	if(Trigger.isAfter){
		if(Trigger.isInsert || Trigger.isUpdate){
			for(Matter__c matter : trigger.new){
				if(Trigger.isInsert){
					oppIds.add(matter.Deal__c);
				}
				else if(Trigger.isUpdate){
					if(matter.Total_Fees__c != trigger.OldMap.get(matter.Id).Total_Fees__c){
							oppIds.add(matter.Deal__c);
					}
				}
			}
		}
		if(Trigger.isDelete){
				for(Matter__c matter : trigger.old){
					oppIds.add(matter.Deal__c);	
				}
		}
	}
	if(!oppIds.isEmpty()){
		for(Opportunity opp :[SELECT Id,Engagement_Total_Fees__c,(SELECT Id,Total_Fees__c FROM Engagements__r   ) FROM Opportunity WHERE Id IN :oppIds]){
					Decimal totalFee = 0;
					for(Matter__c matter : opp.Engagements__r   ){
							if(matter.Total_Fees__c != null){
								totalFee = totalFee + matter.Total_Fees__c;
							}
					}
					opp.Engagement_Total_Fees__c = totalFee;
					lstOppToBeUpdated.add(opp);
		}
		if(!lstOppToBeUpdated.isEmpty()){
			update lstOppToBeUpdated;
		}
	}
}