You can & should set up your funds in a trust, which can be revocable or irrevocable (a lawyer can explain which applies to what you want to do). The trust does not need your sibling's SSN number, as it will have its own number & file its own taxes separate from yours, which is a huge advantage if you are in the higher tax brackets. You are the trustee, with sole power for managing the assets, which get titled in the trust's name. You'll want to specify in the trust document how the assets get dispersed, plus contingent beneficiaries & a backup trustee for yourself. I think it's best to set up a separate but identical trust for each sibling, as disbursements will likely differ as time goes on. While the primary purpose of your trust is for retirement, you may want to specify some latitude for things like medical emergencies, at your discretion. The cost to draft a trust document like this should not cost more than a basic will -- keep it simple.
You can gift your maximum amount for each sibling every year; over time, this will really grow when the investments are set to automatically reinvest. Or you can tap your lifetime gift allowance if you want to front load the investments.
I've set up & managed several educational trusts for nephews & our DS. Doing something similar for growing a retirement fund is no different. I file the tax returns, but you could get a CPA to show you how the first year & just model it until you need to learn how to manage disbursements. Capital gains & ordinary income rules apply in the same way as to individuals, so investing within the trust should be with tax efficient investments that you can run on autopilot.
I understand your desire for anonymity, as the existence of a trust could make spendy siblings even more carefree. As for funding your siblings' expenses now, be careful or you will be increasingly hit up for all kinds of "opportunities" or disasters. In my experience, loans to family members only cause grief in the long run -- they almost always consider them gifts, which is how you should too. If you haven't already done so, read the "Millionaire Next Door" about the perils of adult economic outpatient payments.