This is probably going to be the other way round than you have put it.
You are supplying the deposit and she is supplying the balance of the purchase price.
I don’t know whether you are giving her a gift or whether it’s meant to be repaid but if you are giving her a gift, then provided you live for seven years after making the gift, the gift is removed from your estate for inheritance tax purposes.
You can have rights over the property if you wish, it depends whether you want that. If you want to be repaid when the property is sold, then you can have a second charge over the property which means that you would get paid whenever the property was sold or remortgaged.
There is a standard form for that available from the land registry CH1 which the solicitor would complete and ask you to sign.
You may be asked to provide ID and proof of residence to the sister-in-law solicitor and also, proof of where the money is coming from. That is for anti-money laundering purposes. It is not the solicitor being difficult, it is to stop the solicitor getting to trouble for handling money which is coming from a third party other than a normal lender.
You don’t have to take independent legal advice and the solicitor cannot insist that you do. Ultimately, you are free to give this money to your sister-in-law and she gives it to the solicitor and the solicitor then satisfies himself as to the source of the funds.
Can I clarify anything for you?
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We can still exchange emails. Best wishes.
The solicitor may ask you to sign something which confirms that you have been advised to take independent legal advice but declined to do so. That is not unusual.
What you are proposing to do simply by way of letter should be perfectly adequate for the purposes. If you call into the solicitors with your passports, he could take copies of them if he wishes.
Any document is better witnessed because then it’s more difficult for someone to allege that they were pressured into signing it but it doesn’t need to be and that really is a little overkill to be honest.