Lydia's mother is briefly mentioned in Portrait of Mavis as a Young Vampire when Lydia tells Mavis about how the unicorn sisters stole her ("my mother's") tea set.
It is unclear as to whether this may also be the mother of Lydia's younger brother Drac since it has never been clarified whether they are full siblings (like Bart and Lisa Simpson, for example) or half siblings (like Victor and Valentino, for example).
Any possible relation with Drac's father Vlad is also unclear.
Mavis complains "it's her tea set story again!" indicating she'd heard about Lydia's mom before.
Mavis' conversation with herself and with her friends drowns out most of what is Lydia is saying, although "identical twins" is a phrase heard, probably referring to the unicorn sisters.