What carrier is your sister with? What's your carrier? When my hubby sends a text from his phone I receive it as an email. When I reply it goes as a text to his phone.
Each carrier has specific email addresses to use to send email to text.
If you want to text her from your phone to her email, have her send a text as normal and she can reply with your carriers text address. She will need to address her email to [email protected]