I get the reasoning - you love them and want them to be safe and comfortable.
But the #1 main event question: Have they asked you for help?
Mortgages are one of the best/easiest debts to manage. In general, they can't lose their house unless they bungle their money situation badly. The fact that they have a HELOC on top of still paying off a conventional mortgage is bit concerning just taking the basic facts you presented - mid6os, father on SS and mother works part time and they are just getting by - but is the "just getting by part" (you thinking they're hurting or struggling) just what you think/believe, or have you confirmed this through a review of their debt/income/savings/investments?
It's weird to me you believe they are in a bad money situation, but then also are concerned about inheriting from them. There's a disconnect here. How could they have enough to pass on to heirs if they are just getting by? If they are have their money locked into a house (no real money/investments but very expensive property) then that's actually not that big a deal. Worst case, your mother quits working and their combined SS isn't enough, they could sell the house and move to an apartment/condo/rental property and get the equity out of the current property.
Has your mother even said anything about wanting to retire? Some folks like working part time as long as they're able - not just for money, but because it gives them structure, social interactions, define themselves through their work, enjoy the work environment, etc.
But as the others have mentioned, there are other factors that you can't control, and things not being addressed or that may make this not a "throw money at it" type of situation. I'd start with asking them if they are doing okay and listen to what they say. And then asking them if they think they need help with figuring out financial resources/debts and what is available for them as they near retirement age, and go from there. It may be as simple as they need help with a plan/budget, or all the way up to their children helping out financially. But I would not proceed without their complete openness to put all the info out there along with the invite to step in and help them figure everything out.
And if you do end up offering financial help, you need to make sure whatever you all decide, everyone is aware, there is no ambiguity, no I sort of thing this is a gift, but REALLY would appreciate it if they paid me back type of thing. Work it out like reasonable adults.