Chocolates or traditional local sweets are always a safe bet - I had a flatmate from turkey who bought really cheap Turkish delight and then gave it to people knowing they wouldn't saying anything if they didn't like it in case it offended her.
Agree - chocolates or sweets, particularly "traditional" or "local" ones if you're from a region noted for that (tablet and macaroons if from Scotland, fudge from Devon or Cornwall etc) Or a small hamper for each couple. Even if people don't like or eat sweets or chocolate, they're always useful to have to pass around when you have guests for dinner or whatever.
You could ask SIL if there's anything general they're particularly into or hate (eg are they massive gardeners or teetotal or vegan or diabetic or horribly allergic to everything) and then choose generic gifts which are at least vaguely appropriate.
Gloves - TK Maxx always have decent leather or wool for token prices. Shortbread Sweets/chocolates - PPs' comments about local items are brilliant. Champagne/Prosecco/Scotch/Gin
Definitely some "host" gifts, btw, eg a pot plant/bottle of wine/big box of chocolates.
I would go for edible gifts - Waitrose are doing half price on all their boxed chocolates tomorrow. You could do a hamper of nice things - posh teabags or coffee, biscuits, jams - the sort of thing that comes in handy over Christmas.