If your spouse dies, are his/her siblings still, in the eyes of the law, your sisters-in-law and brothers-in-law? If they are, what if you get married again?
Interesting question that.
My own thoughts are that Sisters / brothers in Law are not a legally recognised 'state' anyway so it doesn't make any difference.
If there is a dispute with your former spouse's will or some sort of financial issue with the estate and they / you go to court over it then they will be identified as your spouse's brother / sister and / or a beneficiary of the will / estate in question.
That is my understanding of it. If you are going through an issue at the moment I would recommend speaking to Citizen's advice where they will tell you, for free, where you might stand.
I think most people would consider them to be your ex sisters/ brothers in law, though I call my brother-in-laws just that even though I am divorced from their brother. It maybe is a personal choice , depending on the circumstance of your relationship with them. if you remarry look on the brighht side, you will have an even bigger family to woryy over!