Matt Barnes -- Get Over Derek Fisher Beef ... Says Ex's Sister

No more threats ... no more fights ... Matt Barnes has to SQUASH THE BEEF with Derek Fisher and get on with his life -- so says the sister of the woman caught in the middle.

TMZ Sports spoke with Laura Govan -- whose sister is Gloria Govan ... the mother of Barnes' children who rebounded with Fisher after she and Matt broke up.

We asked Laura if she thinks Barnes will make good on his promise to fight Fisher again -- a follow-up to their first clash back in October.

Laura's response is measured and thoughtful -- essentially saying Matt shouldn't hold Fisher responsible for falling for Gloria ... because you never know where love is gonna come from.

She adds, "Things happen, you move on."