How long have you led your family up the garden path? - to lead so. up the garden path = ein X für ein U vormachen

How long have you been pretending that everything is all right?

z.B.

Another possibility:

How long have you been hiding it/this from them?

(Leading s.o. up the garden path doesn't seem appropriate to me in this context.)

There's the expression to play s.o. along. No idea if you were thinking of that. I don't think it would be appropriate in this context (health problems), either.

"- the truth comes out.
Have you been "blowing smoke" at the public all along?

http://www.royalcityrecord.com/story_print.ht...

How long have you been blowing smoke (at your family)?

Sorry, Reinhard, but blowing smoke also seems like a strange thing to say in the context of a health issue.

How long have you been fooling them (into thinking ...)?

e.g.

It depends a bit on the pov involved---simply concealing the illness: then SD3's #3; or actively pretending health: then ##2 or 6---another possibility in this latter case would be "to lead so. on": "Have long have you been leading them on?" or, good idiomatic option: "How long have you been kidding them?"

How long have you been putting on a false front?

I'd keep it plain and simple. For example:

How long have you been keeping this from them?