This may not be true in your case, but what I find most often with parents who say "they tried ignoring the conflict" is this: They "ignored" for a short period of time, but then got feed-up, intervened to protect the younger one, and poured on a bunch of intensity while "things were going wrong."
Result:The older sib gets a payoff because she received the parent's intensity by pushing her "anger" button, and the younger one gets a payoff because he received the parent's intensity by pushing her "protection" button. Because of the 'payoff', the "problem" is reinforced ( i.e., behavior is rewarded and therefore repeated).
Ignoring behavior is an over-rated parenting strategy, but in this case it is the recommended strategy -- unless physical violence enters the picture (e.g., throwing things at one another, hitting, pushing, etc.). When the kids become violent, the parent will do best by using the strategy "When You Want Something From Your Kid" [in the Anger Management chapter of the Online Version of the eBook].
I hope this answers your question adequately. If you need any clarification regarding the strategy listed above, don't hesitate to email again.