Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no harm in celebrating two Walimahs (marriage celebrations) or more in a place and at the same time. It is advisable to minimize the expenditure, to avoid lavishness, and also to invite a great number of people.
Therefore, your parents are not allowed to refuse celebrating two Walimahs at a previously agreed time for superstitious reasons.
Their stand or belief that your aunt died for sharing in her Walimah is nothing more than a myth and a bad omen which is forbidden in Shariah. Know that someone's death is not related to the reason of sharing in Walimah since there is no legal or logical evidence for that. The Prophet (Sallallahu Alaihi wa Sallam) prohibited belief in bad omens; he said, "There is neither infection nor ill omen". [al-Bukhari] He also said: "An ill omen is a form of Shirk".
Allah knows best.