Bismillaah-ir-Rahmaan-ir-Raheem.

Wa 'Alaykum-us-Salaam wa Rahmatullaahi wa Barakaatuh.

It is not permissible for woman and her maternal or paternal aunt to be married to the same man at the same time. However if one is divorced, the other can get married to the divorcee of the other, or if one passes away, the other can marry her widower. (Hidaayah)

The Messenger (sallallaahu 'alayhi wa sallam) has been reported to have said: A woman cannot be married upon her maternal or paternal aunt, nor upon her brother's daughter or sister's daughter (i.e while either of them are still married to him). (Muslim, Abu Dawood)

And Allaah knows best.

Wa Billaah-it-Tawfiq.

Wassalaam.