It's unsaid, just implied, how else could she have known Akira as a baby to be able to play peekaboo with him? I guess she could've just been a baby sitter but think she might've been too young at that point, unless she's way older than expected, i figure she can't be more than about 10 years older and i wouldn't trust a 10 year old with a baby so that pretty much just leaves a friend of the family or a relative. Tie that in with marrying Nozomu and there's got to be some degree of spending time with the Arakawa family spread over many years.