How much should I spend on present for sister in laws wedding?

[ssd]

we don't have much money, about £16k a year, so not much spare

wedding is next month, how much should I spend?

Does she have a wedding list or are you just buying something you think she would like?
If buying from a list i would buy the cheapest thing and maybe offer to babysit or do her ironing ,or press her bouquet or do a wedding memory scrapbook or something practical,she will probably appreciate that more than a set of fluted glasses,lol.
I am sure she wouldn't expect you to stretch your finances just for her wedding.