Why did your sister do that journey if she didn't have adequate support?
I'm not saying this to be facetious or anything but many of us disabled people are also parents so we DO get the difficulties of having to juggle a child but we are also highly restricted in what we can and can't do if we do not have someone to support us. It's a highly dependent and undignified life.
Your sister should fold her pram down because had someone with a wheelchair or mobility aid needed to get on and she hadn't they would be left in the middle of a train carriage unrestrained and possibly in a dangerous position whilst they wait for your sister to then, on a possible moving train, have to fold her pushchair down and wrangle a small child too.
Just because nobody needed it right then doesn't exempt her from foresight or consideration.
She chose to be pregnant. She chose to have a child. She chose to travel that day. She chose to travel with that luggage and she chose to go alone without support.
For us disabled people, we often don't travel because the choices that enable us safety or independence are hindered by people being inconsiderate or we have to have a support person with us, and everything is a battle or an argument or a confrontation just to find a seat, or a wheelchair space or a ramp.
Now my disabled child uses a pushchair as a wheelchair because he has no physical disabilities but requires constant restraint for his safety and I couldn't tell you how many times I've had unnecessary conflict with people using accessible spots on buses and trains and trams for their shopping or pushchairs or luggage, and it has made me quite fearful of going out so we only travel when absolutely necessary. Do you know how isolating that is?
Yes it was shit for me as a parent trying to find adequate facilities for my child when he was a baby but it's such a short period of time in life for many but for us unlucky lot it never ends.