In order to configure the multisite feature of Magento, I suppose you created several VirtualHosts in order to access your Magento site when accessing those domains, am I wrong?
In this case, you should modify the configuration of those VirtualHosts, you will only need to add those lines inside each VirtualHost you created in order to redirect from the http site to the https one. Remember that you'll need to restart Apache after modifying the configuration file.
Let us know if that works for you.
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