Loans never make sense by involving family. That you so vehemently disagree and then conclude that anyone holding such opinion must not be good at trading only shows that your thought process is perhaps a little naive and light headed. Perhaps it's just you who is a little young and does not want to hear those sharing their experiences who have heard of or gone through drama that involved family members and debt.
Can Drama be avoided? Sure, write iron clad contracts and have the family members sign and include crystal clear disclaimers that they might lose all their investments. If they sign such thing with you as someone who easily draws conclusions of others you know nothing about then I suspect they treat their "loan" to you as a gift anyway and in their mind write it off already from the beginning.
Any loan always comes with a risk of not being paid back, there is no need for any kind of special contract stating this. This is true if you get the loan from a bank or wherever. Obviously all parties involved in a transaction should always evaluate the risk involved.
Frankly, categorically refusing to even consider loans from family is a loser mentality. You are completely focused on the possibility of a disastrous downside instead of performing a rational level headed evaluation of your opportunities. Families that operate in this manner will have a disadvantage over those who cooperate with eachother rationally because when done properly it is win win for all involved. Family loans have the potential to be much larger and more easily available than what you could get from banks, which tend to depend on dumb computer algorithims for their underwriting.