Ok. So if you're not on the birth certificate, it means you have no legal rights or say in the child's welfare - as no PR. It doesn't mean however, that she couldn't pursue you for Child Maintenance in the future, if she decided to. If she says you're the Father. If you deny it they could say you have to take a dna test. If you refuse to take a dna test they can still bill you (according to Mumsnet! Where a member did just that).
So it's all very uncertain. She might say she doesn't want any cms and then change her mind in the future. So depends what kind of person she is and how independent she is.
With that in mind, would you still want to be on the birth certificate? If for example, the Mother dropped dead when the child was three, you're the other parent. With PR (ie being on the birth certificate) your child would then live with you - unless you agreed for the the child to, say, live with grandparents.
It's a huge pressure having to decide all these things now at this early stage without making things harder later. I would maybe say to the Mother, that you need to have a bit of time to process it all as it's a bit of a shock and you're not quite sure what to think or feel right now, and suggest you have a chat again soon. Thing is meantime she might be thinking too.
In other words don't be pressurised into deciding all this right now. All I know is, once they come along you have this bond and they're yours and they give something back.
I can see from your situation you've seen things a different way, growing up without a Dad. And your brother is an important part of your life. You can do both perhaps though. They might be good for each other. Your child and brother.
So it sounds like the Mother is happy for you to see the child regularly with you mentioning every other weeke-end and a midweek night. That is what a court would order as a standard schedule (plus half the school holidays).
Has your Mum got a view on this? Our parents sometimes have views of their own that aren't always what's right for us - based on their own experiences - but they usually accept our decisions IME. Your Mum would be the child's Grandma. Gets some getting your head round
Thinking about schedules ongoing is good - helps you work out how it could work.
For a baby they do better with only short times away from each parent. So that could be every 2nd or third night until about 6 months. Then a full day and night at week-ends. (ie split week-ends). Once they get to 2 they are fine with every other week-end and a midweek night but it's still better to have two midweek nights or they go a week without seeing you and can get anxious about that.
I had alternate midweek nights - Monday one week, Thursday the next. This worked well as my son was with me every 3 or 4 days. And in nursery during the day.
As an example, this would be a 50/50 schedule
Every other week-end from Friday to Monday (that's one week-end every two weeks) and two midweek nights. So one week your child would be with you two nights, the following week 2 nights plus the week-end. It's not as much as it sounds.
Children grow up with what they know. My son has never known anything different but having two homes. It's normal to him and he was very vocal at the age of 3 as to how much time he wanted in each home and didn't want to go more than 3 or 4 days away from either parent. You both become important to them.
That's looking on the positive side of things, but I know it's a huge thing to suddenly happen for you. And no it's not your fault - it happened - you were both responsible and it happened. Mothers get to say what happens with their bodies though so there's a difference over choice of what happens there.
How old is your brother?