My wife's name was added as a joint holder of the bank account of my son when he went abroad for further studies to facilitate giving instructions to the bank. My son had earned income from Employee stock options and invested them in mutual funds instruments connected with this account. To finance his studies he slowly liquidated these funds. All transactions have taken place through the bank account. By virtue of being a joint account holder is my wife also liable for any taxes on the gains from these mutual fund instruments?