Same question was also in my mind but when one of mine friend subrajit used stayuncle, then I came to know that it doesn't matter what is the relationship between you and your partner, because hotel only asked for the ID of both guest along with the booking mail.
It was so simple he provided his ID along with her partner's ID and they get smooth check-in.
He is very happy happy with stayuncle. Although I didn't used stayuncle till now, but whenever time will come, i will definitely go for it.
God Bless You UNCLESSS!!!!