Does advising a mother not to give anything to an untrustworthy aunt cause strife between the two sisters?

There is no harm in informing the mother about the sister's situation so that she may be cautious, while emphasizing that the solution is not to cause discord, but rather through advice, good counsel, and explaining the danger of treachery; as Allah Almighty says: ﴿Indeed, Allah does not like whoever is a betrayer, an arch-sinner.﴾ [An-Nisa: 107], and His saying: ﴿Indeed, Allah does not like traitors.﴾ [Al-Anfal: 58].