As @LordBuckley said, rather than everyone having to sign up for an account, they can just have you be the test manager.
When looking at DNA matches on Ancestry you may have already seen some matches that say something like "XXX name" and then below it "Managed by YYY name".
This is exactly what LordBuckley was talking about.
The DNA is that of person XXX but it is person YYY who is managing it, they are usually a relative eg a child or legal guardian**.
When you go on to the DNA Matches page, you will see in the top left hand a drop down box that says "View Another Test" which allows you to swap between the different tests that you manage (your's and anybody else's)
If you want to let your sibling see the tree then that is easy to do as well. Once they have an Ancestry registration then you can just send them a Share link. How to do it is explained here:-
https://support.ancestry.co.uk/s/article/Sharing-a-Family-Tree
** Often, the DNA match is an elderly relative (for example, I manage the DNA tests for my parents who are in their 80s). But it's not always like that. I once contacted a DNA match who, it turned out, was an adopted child and their test was managed by their adopted mother (I don't exactly know quite how ethical that is?)