she has to lie i guess. as a 'mother' she would have more say in lucas' future, so she'll be able to protect him more. An aunt would probably not have the same level of authority.
it makes sense though, it's only logical to do that in a place where competition for heir is fierce and she can be easily shunned. A mum is given more authority over a child than an aunt afterall.
Not a huge fan of the development of her lying about being his mother...