At what point do I have enough reserves and should I start considering other ways of increasing financial security?

So I recently got a job that pays per hour logged (online, so no covid-risks), and I'm now earning significantly more than I'm likely to spend (up to 3200 euro per month before taxes)

I currently have my sister financially dependent on me due to a long period of illness making her incapable of finishing highschool at the time and incapable of holding a job as well. She's now remedying this but we're still at least 1 year away from her being admitted to a school with a grant to fund her costs. I'm a Dutch citizen living unofficially in Germany (can't travel back & forth cause covid), I'm not sure what my sister is. I'm in my mid-20's, and have no employment history beyond that as independent contractor.

At the moment we have about 16K euro in reserves, half/half hers mine. Rent is 1000 per month including heating but not electricity etc, total base would be 1200 per month. We're looking for cheaper accommodations, but there's no guarantees. Adding other costs would make about 2K euro per month total regular costs.

I personally also have 35K euro study debt, but if I set up a company this year and make sure my total taxable income remains under a certain level for the next 2 years, I won't need to repay it. There's no interest, as it's a government

Atm I've only got a contract until May, so I'm 'waiting' that one out first, this is mostly on the assumption I'll get a longer employment contract.

My question is, at what point in terms of savings should I start redirecting the money I have to spare into other ways of financial security (investment funds, real estate, etc), and what should I focus on.