Right,if I understood well I'll ask my family member to receive the money in his bank account and give me cash and report "gains",that's the part where the issue is.What gains?I didn't buy Bitcoins in Belgium so what gains should him report?Should he report the amount sold or...?
Usually, if you have gains you have to fill a form and send it once a year to your local IRS, which in Belgium is called General Administration of Taxes. Since you are using your family member's account, that's the "weak point". If anyone will ever be asked where that money came from, it will not be you, but your family member, because in the government's eyes he/she received the money, so he/she had an income, not you. If your family member can't prove how much he/she paid for the coins, the gain is all that they're getting for selling them. As for when they have to report it and on which form, they should ask in the tax office. Usually it's once a year with all the other tax forms and can be done online. I bet you can also ask the tax administration about the details, just email them if you're unsure.
There's also a chance that they won't require any reports since the tax is 0% anyway, but most administrations do.
