Thanks for your enquiry.
If you proceed with the lifetime mortgage, then firstly, your brother in law will have to agree to remove his existing charge (so that the lifetime Mortgage provider gets a first charge over the property) and their consent would be required for your brother in law to place a new charge ranked below the lifetime mortgage).
Please note that equity release/lifetime mortgage providers do NOT normally agree to a second charge being placed on a property. Your financial advisor should be able to get confirmation one way or another from them.
Even if the provider agrees to the second charge, the next question is whether your brother in law would accept this. I don't know how much equity you have in the property, but as you will be aware the amount outstanding under the lifetime mortgage will accelerate and there may not be enough in the pot to clear your brother in law's charge as and when the property is eventually sold.
I trust that you are aware of the implications of entering into a lifetime mortgage-namely that the amount outstanding escalates quite rapidly.
I hope this answers your question, and if so, I would be grateful if you could leave positive feedback.
If you do require any further clarification, please let me know.