just say we really have some catching up to do how about we start over lunch.
You want to go get lunch?
She's your cousin? Have you spoken at all in these last 20 years? If yes, then asking her out to lunch shouldn't present a problem. Tell her you'd love to see her and ask her when can she do lunch. If you haven't even spoken to her at all, then you might want to start with a few calls first to see if she is even interested in reconnecting with you.
Hey we have not seen each other in 20 years. Would you like to join me for lunch on Wednesday?
Hey do you fancy going to get some lunch sometime?
be sweet and be like u wanna grab some lunch?