Disclaimer: Based mostly on my own life/mistakes/situation. Others may have a different/additional/better/worse point of view....
Mentally: If they dump or guilt trip you on more responsibilities to them or to the baby half-sibling, don't fall for it. They're the ones who had the child, not you. At this point, your primary responsibility is to prepare your own self to live as an adult. You matter as much as any other members of the family. If you can talk to them about your concerns, that's great.
Physically: Make sure they don't put the crib in your room; a newborn will keep you awake. Try not to accept even more work from them. If you do things for them or the baby, without injuring anyone you can fake incompetency or no-knowledge (if they won't hit you too much for it), so they don't have even more reasons--excuses--to leave the work to you. If you're teaching yourself something useful like plumbing, driving, cooking, or finances, those are good to know for when you need to live on your own. Though you can still learn yet fake incompetency/lack of knowledge to them. :)
Educationally: Whether university or vocational school, stay on top of your schooling if you can if it's the best route for you to survive on your own as an adult.
Hugs and Take care!