To never do them a favour again?

In that case, I'd tell them that it is obviously causing them and you a lot of stress, they should find someone else to provide this service. You were volunteered for this by your MIL with no consultation and you obviously can't help being ill.

Their reaction is completely unacceptable, and as they won't be grateful for this free service, you might as well piss them off even more and completely refuse to do it.

What does your husband think about his family's behaviour towards you? Hopefully, he will support you and tell them how out of order they are.

What on earth was OP supposed to communicate? Was she supposed to say "By the way, if I'm unwell I won't be forcing myself to do this massive favour for you till I'm well again, and I will have to put my contractual obligations to my paying clients first"? Surely that should be bloody obvious?

YANBU
The response you’re looking for is “If you want me to do this work for free then you can wait for me to do it. If you need it sooner, feel free to pay someone else”

I'm not even going to ask why your DH has spoken to both your BIL and MIL. The answer is always the same... He doesn't like conflict, his mother is overbearing. If that is your family situation, you need to tell him that the conflict that might occur if he politely explains to them what selfish, entitled, cheap fuckers they are being (and he can add interfering for his mother) will be way less than if you have to point out how fucking rude they are and give all his paperwork back to him unless he pays £5k up front. That is an arbitrary amount but I wouldn't do a few days' work for people like that for anything less.

If standing up for you isn't too uncomfortable for him, ask him to return all the documents to his brother.

"Sorry a deadline was never discussed. As I am doing this for free it will be completed after my paid work. As I have been ill there is now an extra delay. If SIL needs it ASAP I suggest they find someone else as I cannot meet that deadline unfortunately."

I wouldn't be doing anything for them in the future, you were ill and even mil is sticking her oar in.
My ususal response to cfers usually means that they avoid me like the plague as do any of their hangers on. I don't tolerate idiots or twats randoms, strangers or family.

this 👆

well what happens if MIL offers your services again for free?

Send something like this to them, your in-laws are CF.
'When i was asked to do £(insert your usual fee ) for free no deadline was stipulated.
You gave me the information neaded X working days ago. As you know I have been very ill for X of those working days and unable to work.
I do not appreciate being bad mouthed and harassed regarding completion of £ ( usual fee ) for free when I have been ill.
I will endeavor to complete as soon as possible but im sure you'd agree going forward it is best you employ and pay someone to do this work.'

I wouldn't do anything again for this lot.

Cheeky bastards!

I’d send them all a message saying something like this and never do them anything for free again.

As you’re all aware I’ve not been well and have been off work sick. A deadline for this free work wasn’t mentioned when I said I’d do it as a favour and the paperwork was delivered.

I can complete this work by X date, alternatively I can suggest a different contractor who may be able to complete earlier.

Their fees will be around £x which would be my standard rate had this not been offered by MIL as a favour.

This.
You had the paperwork last week. Just return it. They can pay if it's such a rush.

If it’s urgent you best get someone else to do it. Give him his stuff back and don’t do anything for free again

'I wasn't told it was urgent. If it is, they I can recommend someone who will do it for a fee. If it isn't, I'll get to it once I am better an I have dealt with the backlog from my paying customers.'

I would crack on today and get it done if you can and then send out a family message to all saying from now on you won't be doing work for free as they've took the piss knowing you were unwell. Who cares if it starts world war 3 they are cfs.

tbf even if it’s a favour, you should have communicated better about expectations and he absolutely should have told you about the deadline.

This - or he will have to pay a “rush” fee if it is urgent.

Don’t be a doormat tell them you were never told it was urgent and I don’t appreciate being hunted and accused of not doing something when I was providing my expertise for free.

Age old saying your lack or planning is not my emergency!

He should have told you the deadline. Same as if he paid for it, but even cheekier as he didn't. Say that to MIL, and ask her why she is listening to SIL and bothering you, instead of B/SIL sorting things out themselves.

I’d hand it back incomplete and say I was too unwell to make the deadline. Telling him to shove it up his arse optional.

“As you are aware (as I missed the family meal at the weekend) I have been unwell. I have been unable to undertake any work until today, paid or otherwise. I only received the paperwork from you/BIL on x day, with no mention or discussion of any deadlines. As you will understand, being self employed yourself and running a small business, I have to prioritise paying clients. As previously discussed I will have the work to you by x day. Should you need it sooner there are other professionals who will be able to assist you for a fee”

Then if you’re asked in future just simply say you haven’t got time, you’re no longer doing favours, or you’ll do it for a discounted rate if you’re feeling gracious.

Stand up for yourself op. Far too many ignorant people about these days taking advantage.

If someone is chasing you for work you are doing for them FOR FREE, they deserve to be professionally told to get absolutely fucked and take their paperwork with them!

And as for MiL, she just deserves to be told to get fucked and to never mention your name again.

Pair of CHEEKY FUCKERS!!!

I haven't voted because you are being unreasonable to do it at all, especially now. As PPs have said, I would pass it back to him and say that having been off ill for a week you already have a backlog and can't meet his deadline. That's the professional response. Keep professional boundaries with family and friends, they will respect you more if you do.

You need to reply to your MiL, copying in SIL and BIL, pointing out that you were not told of any deadline, they know you have been off sick and therefore you do not understand why you are being chased for this piece of work. Point out that, if it was so urgent, he could have gone to someone else and paid them to do it at the time he wanted.

This is well out of order. “I was not aware of your deadline. I have been unwell but will complete it as soon as I can.”
I would not respond to other family members.

No I haven’t completed the FREE work I said I’d complete while recovering from a bug, and in time for a deadline I was never told about. Thank you for the multiple messages of concern to see how I am and all the gratitude for my kindness.