I'm more interested in how the OP finally got into the glamorous front room by 'happening' to call when her brother was home alone. She says
when I got there brother was in well we went in front room- it’s like something out of the bloody Ideal Home Show!!!
but it's not clear whether her brother invited her into the front room, or whether the OP demanded the secret key be handed over and let herself in, or whether her brother regularly uses the room himself when he's at home alone, so the door was just open... I quite like the idea of the OP hurtling through the front door once she realises her SIL isn't at home, shouting 'QUICK, BRO, OPEN THE ROOM! I HAVE TO SEE THE ROOM!'
Also, for all the disappointed sex dungeon people, bear in mind that the OP first realised the room was locked a couple of years ago, and the last time there was any possibility of going into it was last Christmas holidays. Now the room is beautifully-decorated and empty, but it's not outside the bounds of possibility that it used to be a sex dungeon/hoarder's paradise/home to a collection of creepy Victorian dolls, but the SIL and brother got worried when they realised the OP had become fascinated by the locked room, and got rid of whatever was in there...?
OP, had you never tried peering through the net curtains? That seems unenterprising of you.