I'm not a lawyer but I'm thinking maybe you just need to transfer the property once.
If I'm not mistaken, the title or S&P is still under 'elder sis', and your bf and 'lil sis' are beneficiaries through will or something. I think it should be possible for 'lil sis' to relinquish her rights to the property. And you could check with lawyer if she could reassign her rights to you instead with cost (I don't see you mention this anywhere... why?). Also the property loan is under whose name?

On the 6k transfer fee, ~3.5k is probably for stamp duty. You should get a breakdown on the 6k for better understanding.

Frankly I don't really see the need to have your name on the property and/or take marriage into the situation, both are not the true issue which is your bf's financial situation and secondarily your financial situation.

Your bf actually inherited a debt unless it is sold off in which case there might be some cash return. If your bf just need some help for a couple of months, it's no big deal to help a little, and let him pay back later. If he cannot sustain the property on his own, then perhaps it is better to sell it off. There's always opportunity to buy another in the future rather than to get saddled with too much liability.

that's nice to know, thanks.

I gather from your answers that your bf can just have the house sold and the money gain would be more than enough to repay his personal debt and no strings attached.

If you and your bf 'bought' the house over, you just transferred the debt into you and your bf's name --> that's why I said you guys inherited a debt. Whether or not you guys should commit to the house for the next 20 years or so is really up to yourselves. It's not just about your relationship and marriage. It's also about being financially wise and planning for the future.

Offhand I would say it is better to utilize a housing loan to cover one's personal debt, but he must know what he is doing.

Well the obvious question is does your bf have the same financial thinking and willingness to commit as you are? You'll need two to clap to be successful in this game.

And about the debt thing, I view a house that doesn't generate income as liability but it's more a matter of personal POV (point of view) and so no offense taken.