I think what you'll need here is something carefully picked, not too intimate, but personal enough to show you care.
Here's what I picked
Under £50An Art Deco brooch found on Etsy. Period jewellery is a great way to show you care, pick her favourite era, and start the research!
Big SpenderStylish but neutral, those Phillip Lim 3.1 Jeweled Leather gloves from Net à Porter are a perfect gift for your mother in law. They will add a fashionable touch to her outfits without being over the top, and they go with any style. Leather gloves in general are always a good option.
Don't forget to have a look at my suggestions for your BFF, your sister, your mum & stay tuned for gift ideas for your grandma, aunt and the men in the family!