(England) Company reducing pay/hours -

Posting question on behalf of my siblings who work at the same organisation. They have both been employed over 2 years and company /employees are based in England. Company is family run and a small org.

Company is currently seeing a drop in work. For Sibling 1 (S1) they have said they will be dropping their wages by a certain amount per week. They have not had my S1's agreement. Contract has been described as limited but sounds like sibling hasn't even seen the contract, or may not have a written contract (not clear if they have a copy at the moment). S1 will be raising a grievance. I've advised them to contact ACAS. S1 feels they would be better off with redundancy (they have decent length of service).

Is there anything else S1 should be advised to do in the meantime? Not sure if this is orgs tactic to have them leave without going through redundancy process eventually.

Sibling 2 (S2) has had their hours/working days dropped. Told its temp until work picks up. Like S1, the contract may not exist or is limited. Doesn't sound like any of this has gone through in writing. S2 is pretty naive with employment rights unfortunately and verbally agreed to this drop in days, however I believe they did so without understanding the full implications, believing they had no other option. Is a verbal agreement seen as acceptance of contract revision? Is it too late for S2 to raise a grievance?

Although I don't have contract details, I'd be surprised if it contained clauses on lay-offs/short term working. Is there any other advice here other than them going to Acas? It seems they will potentially lose their jobs eventually and I want to ensure they get their full redundancy pay if that were to happen (not based on new enforced hours/wages).

Apologies if formatted poorly as done on a phone.