Swear that you will love Miranda and Isabelle for as long as they live

"I will love both you and Isabelle for as long as you are alive," you say firmly.

Isabelle's voice sounds softly in your ear. "But how can you be certain?"

"My brother of mind will know a way to make certain of it," you reply. You guide your mates over to your brother of mind's dwelling, where for once he is alone.

"Hello, brother," he greets you. "What can I do for you?"

"My mates are worried that I won't love them when they age," you inform him.

"I do have one thing I can use for that," he tells you. "But... I've never used it before. The witch who gave it to me let me know that it can have... serious side effects."

"What sort?" you worriedly ask.

"You must make a solemn vow," he tells you. "If you ever break the vow, you will age a year for every day you live, until you find a witch and take your mates with you. The witch must grant you forgiveness, as must your mates in her presence." He pauses. "And witches hate males."

"I'll make the vow," you say.

Your brother of mind has you and your mates sit on the floor. He draws strange shapes around you and takes a sheet of parchment down. He reads the words on the parchment, then hands you a vial of pink, bubbling liquid. "Swear that you will always love your mates."

"I will always love both Miranda and Isabelle until the day of their death," you say. You drink the liquid.