"Lisa will not be able to go to the wedding. They have reached out to send their condolences, and accept she needs to step away from filming the show right now so she can attend to her family," a source told RadarOnline.com.
Jax, 39, and Brittany, 30, are set to tie the knot at The Kentucky Castle in Versailles, Kentucky, on June 29.
The former Real Housewives of Beverly Hills star will be flying to London to help plan her mother's funeral.
Her loss comes a little over a year after her twin brother Mark Vanderpump, 59, committed suicide.
Mark was found dead in his home on April 30, 2018 from a suspected overdose.
"She’s absolutely devastated at losing her mother when she is still grieving over the loss of her brother," her rep told the outlet.