I've found Seven Til Midnight sizing to be generous, unlike many other brands where I find you have to buy at least one size bigger to stand a hope of it fitting.
To minimise the risk of buying her something that doesn't fit, I'd definately recommend STM range.
Hope you find what you're looking for!
If it's PJ's you're after now, though I know I shouldn't be recommending other places, shops like Debenhams and M&S often have nice ranges of PJ's that are sexy, think M&S has an 'Autograph' range that's brought some nice gift nightwear out for Christmas!