You can install our WordPress plugin to synchronise your WordPress subscribers with your SendinBlue account and send newsletters. To do so, we invite you to refer to our step by step guide.
In addition, installing SendinBlue plugin for WordPress should not interfere with your current MailPoet plugin. We just ran a test with both SendinBlue plugin (Version 2.6.8) and MailPoet plugin (Version 2.7.5) activated and our plugin sent a newsletter smoothly.
Should you have more questions, please do not hesitate to send an email to contact(at)sendinblue(dot)com.
Regards,