If the split involved any kind of abuse or violence, it may be genuinely necessary for your sister-in-law's peace of mind to avoid any reminders of her ex. This might just mean taking down the painting and the wall art when she's over. Of course, you wouldn't have to but it would be the kind thing to do.
But if your sister-in-law's problem genuinely boils down to her wanting demonstrations of loyalty, you're well within your rights to draw a line. It's your home; you are allowed to decorate it (and your own bedroom!) as you see fit.
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