How do my parents secure their assets so that if one of them passes away the other doesn't make bad decisions due to pressure from one of their children?

They need to make a will and/or living trust that spells out how they want their investments handled if one should pass before the other. At some point they should give someone, that they trust, their Power of Attorney (both medical and financial).
Is there someone in the family who is pressuring them now for loans, money, etc.? You may want to take a look at their finances and check bank accounts, etc. for irregularities. You can also place a fraud alert on their credit report that will prevent someone for taking out a loan or credit card in their name.
It is really difficult to keep someone safe from greedy family members if they are "enabling" them.
good luck