Ask her if she morphs into one of your family members.

"Um... 'mom'. Could you um... transforms into someone else, like er... one of our family members?" you asks your shapeshifting girlfriend/mom awkwardly and shyly.

"Of course, sweetie. I could do that." your transformed girlfriend/mother quickly answers. "Who do you want me to be, sweetie? Your sister, your aunt, or your cousin?"

"I don't know, 'mom'." you reply. "It's up to you, 'mom'. Whoever you are, I would like to have sex with you as always."

"Even if I'm a man, sweetie?" she asks.

"Even if you're a man, 'mom'. And I will always love you." you reply back to her again as you kiss her once more.

"Teehee, you're always been my weird sweet boy sometimes, sweetie." your girlfriend/mom smiles and kiss you again. "So without further to do, shall we get going?"

With that, your shapeshifting girlfriend/mom gets up and walks back as she's about to morphs herself, you starts to wonder who will she going to be when she morphs into one of your family members.