An 18year old is old enough to be paid to look after other people's children - in which case it would be a job, so they would (probably) take it seriously and - importantly - be getting paid for it, and probably have some support from more experienced workers, if in a nursery rather than a nanny setting.
An 18year old is old enough to have their own baby - that they have chosen to have and to take responsibilty for.
Both are different to even the most mature, responsible 18yo being asked to act as a long term unpaid carer for their younger siblings.
And I'm pretty sure she wouldn't be able to claim any childcare costs to pay towards a family member helping out.
And he will either get a job, or be forced into training schemes / unpaid work experience at some point, so it would be daft of her to rely on him for long-term childcare.
That said - she may be more entitled to childcare help than she realises. It can seem insurmountable before you go back to work, & it's hard to get reassuring info from jobcentre staff, but there are (or were 
) grants to do things like pay the first few weeks childcare, HB run-on etc.