A good reference for etf vs mutual fund: https://www.bogleheads.org/wiki/ETFs_vs_mutual_funds

My advice is start with a four fund Vanguard portfolio based on your age (use their target date retirement funds as a guideline, e.g. VFFVX). Probably don't want the target date fund in your brokerage for a few reasons, so buy the individual funds and rebalance yearly. Start with mutual funds because they can convert to ETFs, but not the other direction. And because they're easier to buy and sell, no bid ask spread.

If it's for your siblings, hard to say whether you should max out your retirement funds (401k, IRA, Roth) this year or leave it in the brokerage. Retirement funds can be pulled from via SEEP but it's very inflexible. The way the trust is written, possibly just leave it in a brokerage for now. Careful as a trust has legal implications. Might want a lawyer and CFP to review your case.