Hmm your elderly aunt isn’t a close family member of your son so I’m not sure the school will find this acceptable. But you could ask?
Aunts and uncles are often close to their nieces and nephews ime.
Even when grown up they can be.
Whilst is a step removed for the child, that doesn't mean there is no closeness. All families are different.
Besides it is a close relationship for the opposite, and if she wants/needs to go then her child may well need to go with her.
Many schools, my own include, show empathy and understanding and would authorise such a visit. Infact I know we have in recent weeks, as well as authorising time for the funeral afterwards. We have also authorised longer time off for the child to go,abroad in similar situations.
At present schools still have some flexibility in these things and there are still some senior leaders who demonstrate warm, compassion and empathy and try to reduce additional stresses for grieving families.
And even if it is unauthorised - so what? In most schools, there are no fines until day 5 so just go ahead regardless.