You’ll need to open a UTMA or UGMA (Uniform Gifts to Minors Act) account depending on your state. This can be done at any bank or brokerage. You won’t be able to add to it completely for free, as those firms at the very least charge trading costs. So, you could have a per trade charge of somewhere between $4 - $20. This is much lower than professional brokers would charge.
Anyway, that kind of account is the property of your niece, however as she is under 18 an adult must supervise and make the decisions. You need to be aware that such an account is an Irrevocable Trust in the sense that if you or anyone else puts money in, it cannot come back out. Only your niece, after age 18, is allowed to make withdrawals because it’s her property.
We’ll want to enlist the services of a Social Security attorney regarding your benefits question. Don’t take legal advice from people who aren’t lawyers, myself included. I can only tell you what I’d do.
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