Is it permissible to send messages of remembrance of God and admonition to my female relatives, such as my cousin (daughter of my maternal aunt or paternal aunt) and the daughters of my paternal uncle and maternal uncle?

The default ruling is the permissibility of communication between non-mahram men and women when there is a need, provided there is no seclusion (khalwa), safety from temptation (fitna), and no softness in speech, based on the Almighty’s saying: "Then do not be soft in speech, lest he in whose heart is disease should covet, but speak with appropriate speech." There is no objection to sending messages if they are for a need, advice, or teaching, although it is preferable to avoid this as a preventative measure (sadd al-dhari'a), especially if the electronic messages are not seen by anyone else. It is not advisable to send private messages to non-mahram female relatives, even if they contain advice. Instead, religious books and audio recordings can be sent, as this is further from suspicion.