as you would need two rooms and you're going to london and paris and you're looking at the "fifth night free" offer, you're going to need a boatload of points for that. that'll be two rooms x four nights x two cities for a total of 16 nights. assuming you can get a 12,000 point award night, that's 192,000 points.
Are you aware that the "fifth night free" award is for a single stay, i.e. five nights in one hotel?

have you considered the "points and cash" options (if they're available?)

and to the other poster on this thread and the "creepy" or "weird" comments, the OP didn't come here to be judged. I wouldn't want to stay with my in-laws either but I don't know the circumstances or situation of the OP and those kinds of comments are not kind. It's much more constructive to stay on topic.