a user changing companies to our sister company. Different domain, Different location.
I was going to set the forwarding address to @sistercompany.com .. but will emails get caught on spam filters? because the reverse dns of that ip will point to our system and not mail.sistercompany.com
If I understand you, you are forwarding incoming email for the user from your server to the sister company address — this should not be affected by spam filters any more than an email from you to your counterpart in the other company.
Outgoing email replies will come from the user’s new address, i.e. the one you are forwarding to.