The sibling would take over the full mortgage payment and I would step in if needed.
What you actually want to do, it seems to be, is to be a guarantor — you will be liable to make the repayments if and to the extent that the primary borrower fails to. You'd be offering yourself as an additional security to the lender — if the borrower defaults, as well as being able to look to the mortgaged property for what is due to them, the lender could look to you.
But I think that's unlikely to be acceptable to a lender. Lenders don't want to have to enforce securities; they take comfort from having them in case the borrower defaults, but they prefer the borrower not to default, and they generally won't lend to someone who they think can't service the loan, no matter how good the security offered.
So, instead, you are proposing to be be a co-borrower. You and your sibling will be jointly liable for the full amount of the loan; how you handle that between yourselves is a matter for the two of you but the bank doesn't care. The bank should assess the loan application on the basis of whether, between you, you and your sibling are likely to be able to service the loan.
But the bank will still be leery of the arrangement. The loan will be used to purchase a property for your sibling alone — you will have no share in it. They'll be aware that you think of your position as being akin to a guarantor, more than a borrower. They'll also be aware that you are borrowing your share of the loan effectively to pass it on to your sibling, which will leave your sibling servicing a larger loan than they reckon your sibling can comfortably do. So they'll be looking at the possiblity of having to take action against you to recover a loan that was avanced to you but that you, personally, derived no benefit from. If they recover the money from you, the upshot would be that your sibling ends up owning the house free and clear, you end up paying for part of it; you have a claim against your sibling, but no security —
you don't have a mortgage over the property. Those proceedings will foreseeably get very messy as you, e.g., join your sibling in the proceedings and argue that it would be fairer if the bank enforced its mortgage rather than looking to you. Banks don't like mess.
I agree with Brendan. This is definitely a case where you should use a mortgage broker, who can advise on how to present this to potential lenders in the most favourable light.
You and your sibling should also discuss, and reach agreement on, what happens if, God forbid, the bank does come looking to you and you have to pay out money. What are your sibling's obligations to you in that circumstance? How, if at all, are they to be secured? You and your sibling need to get this in writing before the loan is drawn down. I think you will need a solicitor to help you think through this and document it properly.
