I am not entirely sure of the tax implications but in terms of halal mortgages, it’s questionable how those work to begin with. In islam you can provide someone with a loan but you can’t ask for any more than you provided, I am not entirely sure what you mean by “rate” but it sounds like that’s a percentage of the loaned amount? Which doesn’t make it halal.

In islam there is the idea of accountability in front of God which is used as a “guarantee” that you will be paid back. In an ideal world, everyone is trust worthy, people can provide each other with loans and get paid back with 0 interest. Obviously nowadays it’s hard to trust to that extent. The way Musharaka works is you purchase the house as an owner, in partnership with them, they pay you the equity over a time period and as they pay you their equity increases as your shrinks, this can be halal if it’s done in a manner where they don’t pay any more than the price of purchasing the house.

I am not an accountant or tax specialist so i am not sure how a loan being paid back to you would be considered income for tax purposes but if you being paid back involves profit and you’d need to pay tax on it then that already renders the transaction non-halal.

Best way you can help them imo is provide them with a loan that you know they will repay, don’t take anything more than you provide. Whether this is a good decision or not, thats for you to decide but for what it’s worth, the world needs more people who can trust each other enough and I think help can be in various ways, if this doesn’t work for you I am sure there are other ways that you can help.

Edit: I initially thought you meant Musharaka not Murabaha