Offer your thoughts or opinion on the input financial query or topic using your financial background.

Is it legal if I'm managing my family's entire wealth?
All the other answers posted thus far discuss matters from the perspective of US tax laws and are unanimous in declaring that what the OP wants to do is indeed a very bad idea. I fully agree: it is a bad idea from the perspective of US tax laws, and is likely a bad idea from the perspective of Indian tax laws too, but what the OP wants to do is  (or used to be) common practice in India. In more recent times, India has created a Permanent Account Number ("PAN number") for each taxpayer for income tax purposes, and each bank account or investment must have the owner's (or first-named owner's, in case of a joint account) PAN number associated with it.  This most likely has decreased the popularity of such arrangements, or has led to new twists being used. The OP has not indicated the residence and citizenship of his family (or his own status for that matter), but if they are all Indian citizens resident in India and are Hindus, then there might be one mechanism for doing what the OP wants to do: apply for a PAN number in the name of the Hindu Undivided Family and use this number to carry out the investments in the name of the Hindu Undivided Family.  (There presumably are similar statuses for undivided families for other religions, but I am not familiar with them). There are lots of matters here which are more legal questions than personal finance questions: e.g. if the OP is a US tax resident, then the family presumably will not be able to claim Hindu Undivided Family status since the OP has been divided from the family for tax purposes (or so I think). Even if HUF status is available, the OP might not be able to act as the pater familias while his father is alive, and so on.  Consultation with tax lawyers, not just chartered accountants, in India is certainly advisable.