are you being deliberately obtuse about why women might not want that for their daughters?
I have repeatedly stated I understand why someone may not specifically want that lifepath for their child, my point is that you should support and treat teen parents with respect even if you don't agree with the decision to have children (or avoid an abortion). Many people don't do that.
I'll answer your questions though.
what is the combined income of your sister and her partner?
I have no idea, I wouldn't ask them that. If I had to hazard a guess, with the two of their incomes combined I think they'd be making around 30k which would be liveable (if uncomfortable).
can they afford to live independently (ie not with parents or family or in a council house or other accommodation partially or fully government funded?) where are they living now?
They live with my parents at the moment, she's taken some time off work to focus on her son and her college work. AFAIK they're trying to start renting sometime next year but my mum was happy to let them stay and didn't want to kick them out with a newborn. They're also paying for all baby supplies themselves (and to be fair my sister wouldn't be moving out this year anyway, irrespective of baby).
how long have they been in a relationship - is it stable?
They have been together about a year and a half. In terms of stability it has it's ups and downs like all relationships, but they're not reckless. I think having the baby has made them realise they can't fuck about with any drama like their friends can.
can they afford to purchase themselves all major items the baby needs (I know some will be presents)
Yes, they're buying everything the baby needs (a few exceptions being big items like a carseat that you can't buy secondhand, those were bought by other family members as a present).
do they have an education to allow them to get better jobs in the future to support bringing up their child?
She is currently doing a college degree so it will help her get into her university of choice.
has your sister had the opportunity to travel, meet people and expand her education and would you not want that for her?
Meeting people, yes. Expanding her education, yes she's doing that now. Travelling not yet, but if she decided she wanted to travel in the future I'm sure she could figure it out.