Not sure how I wanted to approach this one... but it was asking to be addressed. Anyway, based on some of Isabella's dialogue in certain Christmas episodes, a situation like this has been made apparent to me. Feel free to interpret the situation however you want; this is just mine.
Phineas grinned amusedly seeing the menorah in the front window, and gawked when he noticed the small tree in the corner.
"Is-Isabella?"
"Yeah?"
"Um, interesting question."
She quirked an eyebrow.
"Is that - a Christmas tree?"
She blanked. It obviously was; "No, it's a cactus. Yes. It is."
His brow furrowed. "But I thought-" he squirmed a bit. "Aren't you, um, Jewish?"
Now she understood. "Oh. Hah. Well, my dad is. But mamá's from a Christian family in Mexico. We kinda do both."
He chuckled. "Isn't that confusing?"
"Yes," but she was laughing. "...Yes, it is."