How Do you Tell Your Mom That You have a older girlfriend that she knows i think she won't approve of it?

I Have An Older Girlfriend the problem my mom knows her and i Think That Mom Would Not Approve of my new girlfriend If It Was Some one else and not thiss girl mom will approve but i don't think That Mom Will Like The I Deal Of Me And Thiss Girl What Should I Do? My Girlfriends Brother And Sister Are My Next Door Neighbors