Legal relationship to grandchild's half-sibling

Maybe GSM will be along, but I have dealt with this situation, so will give you an idea (with the caveat that peoper legal advice may be necessary)
Legally, you are a family friend. Occasionally, in Social Services / Family Court situations, your relationship may be regarded as 'close'.
If your situation doesn't include these, then I believe you are clear - but depending on the exact nature of the document, you may wish to take legal advice.
If you have legal advice included in an insurance policy, or trade union membership, you can get advice easily & quickly.
I would add that this child is lucky to have an 'unofficial' gran to take an interest in them.

this has come up on MN, and there did not seem to be a consensus.
it is quite an arcane area, and the answer may vary depending on the exact document to be signed.
best to take advice from the relevant officials.

You are not related to the older child, but if someone chose to be awkward they could argue that she/he has been treated by you as a child of the family and that you are not a truly independent witness. It’s better therefore if they find a witness who isn’t quite so close, though not legally essential.

Trust GSM to come up with the goods!

Thanks MOnica! I do my best. 😊

Thanks everyone, good advice!

Free legal advice.