https://www.cnet.com/news/google-assistant-gets-to-know-you-with-personalization-updates-at-google-i-o/

You'll be able to organize your info under a new tab in settings called "You." You'll be able to find this tab both in the Google Assistant and Google Home app. The tab shows your family and top contacts, and you can add details such as identifying that a contact is your brother or sister. Fill out info such as address and birthday, then you can ask your Assistant on any device about the weather or traffic by your sister's house.

You can even set a reminder to get a gift a week before your sister's birthday and it will find the date and set a reminder accordingly.