Thinking more of the Sister in Law thing, we KINDA have one already. Secretly though. So secret not even the sister herself knows. At least I don't think she knows, can't remember the details.
In non gibberish, during her wedding Veena said her vows to us, not her "husband", we each consider ourselves married to the other. Veena is Priya's sister, so Priya could be taken to be our Sister in Law.
Now all we need is to marry Priya is some way and that'll be the collection complete. Not as openly as I'd like it, but not as openly as it will be eventually either