I get anonymity, but it didn't really make sense - It would have meant when you posted 2 weeks ago about your baby being 2 weeks old, giving a pretty detailed timeline, including a few visits to hospital, getting into a routine and doing ok and then your SD coming for 4 days, that was all the first week, which doesn't really add up. Hence why I think your baby is older than you are saying. But in any event, if baby is 2 weeks or 4 weeks (or 6m) it makes little difference. If SD is 14 not 13, then it makes little difference, as there's nothing wrong with her being in the room either way.
I have no idea why you think I have an issue with your being a step parent - I have none. I'm not a step mother, not a step child, I'm not a single parent. I've got zero beef here. But it just doesn't sound as if you are treating her as a teenage girl of the family to me.
I occasionally looked after my nieces and nephews as tiny newborns, knew how to give a bottle, wind them, pick them up safely etc. The idea of not even getting trusted to be in the same room, like I'm a toddler or a dog, is baffling to me.
Yes, it was the, 90's and people weren't so hot on risk, but at 13 I helped out in church and had my own Sunday school class of 20 kids, which I looked after by myself (sometimes I had a helper in her 80's). Baffling now how anyone thought that was alright though. Until recently, teenage girls were the main supply of babysitters! And older kids are a lot harder to look after than a newborn.
But it's upto you. If you prefer to cart your baby up and down stairs when you pop to the toilet, because you believe that the likelihood of you tripping and dropping baby, is less than her, IDK spontaneously combusting by being left unattended or with a teenager for a couple of minutes, then that's your choice.