The Labour frontbencher told podcast host Paul Brand how he "distinctly" remembers as a school child "spending nights in the high dependency unit" as his mum battled Still's Disease - a condition that causes the immune system to attack itself.
"For the last ten years or so of her life, she couldn’t walk, she couldn’t really talk, eat or really move," he said, but she was "very, very determined".
"From the moment she was told she was not going to walk again, she decided she was going to walk again".
He says the experience of seeing his mother with Still's Disease inspired him to be "very determined" himself.
The Europhile MP also revealed how he's in the dark about the future of his role as shadow Brexit secretary.
When the UK leaves the EU on Friday January 31, the Department for Exiting the European Union will cease to exist and so will the role of his counterpart, Brexit Secretary Stephen Barclay.
"I keep asking Steve Barclay what he's doing after Friday," Sir Keir said, adding how his opposite number isn't sure what will come next.
He said his any new role "largely depends where Steve Barclay goes and what department he goes to" - but Sir Keir knows what job he wants when the Labour leadership election ends on April 4.
He wants to replace Jeremy Corbyn opposite Boris Johnson in the Commons and despite signalling he will adopt a similar stance to the current leader, he refused to definitively rule out pulling the party back towards the centre.
He said the "radical changes we have made since 2015 have been right" but said the party's current position would be a "foundation block" to build on.
He added: "I’m repeatedly saying that the shift we have made in the last 5 years is the right shift, that the 2017 is the foundation document but we need to build on that for 2024."
Sir Keir, who has represented Holborn and St Pancras since 2015, also defended his pro-EU stance that led to Labour adopting a policy which called for a second Brexit referendum - a plan that many say resulted in Labour's catastrophic election loss.
So it will come as no surprise that Sir Keir doesn't plan on celebrating Brexit Day.
"It won't be a moment of celebration. I won't be marking it. I accept what's going to happen," he said.
"My diary is completely controlled by my team now. They will probably have me in a room or a bar practicing for the hustings the next day so that’s probably what I’ll be doing."