You can ask them to at least keep it away from you.  They can have
their cookies, but maybe they don't have to be "community" cookies.
If you don't ask, you're to blame, not them.

Find a substitute, and keep it on-hand all the time.  If you want a
cookie, have the substitute.  As long as you pick something "better"
than cookies, this mitigates the damage.  You ARE still taking in
calories, but the net effect is a reduction.  Or, if you find you're
craving at the exact same time, work in a MANDATORY snack into your
daily diet.  I hit a wall early this year, while my fledgling diet was
only 10lbs in, and by just adding a low-calorie yogurt with a handful
of granola (~100 cal) at 9PM, I satisfied the late-night beast without
hurting my waistline.

If you just can't stay away from your family's bad habits, you can
still help yourself a little:

Decide how much you'll eat, put it in a plastic baggie, and when it's
gone, keep the baggie in sight.  You're less likely to refill the bag
than you are to just grab a snack (or two, or ten) on the sly.

You don't have to move out to be an independent eater, either.  If you
can afford it, start shopping for yourself, and eat [partially]
exclusively from what you buy for yourself.  My family 'tries' to eat
healthy, but when I'm around them, I just can't manage a proper diet.
So I just started buying my own food and eating that.  You don't have
to replace all your meals, either.  Replace the most damaging portions
of your family's diet with a more reasonable alternative.

Most importantly, do not blame your family for your problem.  They are
their own individuals, and they have the right to decide to eat
whatever they want.  They might choose to make things hard for you,
and for that, they're pricks.  But you RECOGNIZE the problem, and
there are obvious solutions.

I know it's a harsh thing to say, but you are the ONLY person
responsible for your diet once you're aware of it.  All of us are
weak, and overcoming our weakness is hard.  But if self-control was
easy, losing weight (or being a professional athlete/musician/etc.)
would not be quite as impressive, would it?

Don't get down on yourself.  If you sneak a cookie, it's not the end
of the world.  Don't let that shame drive you to a second, a third,
etc.  Pull yourself away from the situation, analyze it objectively,
and convince yourself that there's a very easy solution.  Then DO IT.

Best of luck!

I live by myself, but I have exactly the same problem regarding junk
food and things like cookies. Give me 6 months to live, and I'm going
to eat chips, cookies, pies, ice cream and all the rest days in and
days out until I blow up like a balloon.

What I did is turning to "high-fiber diet". I had 36 grams of fiber in
only 1210 calories today, and not hungry in despite of that. If had
that with little fiber, I would be starving by now and looking for
cookies! You have nothing to lose by trying. Leave cookies out for two
weeks, switch to the high-fiber diet, and see how it goes. After
filling you up with fibers, I don't think you'll even want those
cookies anymore. Check out Youtube.

Think of it in these terms - That's a pattern of addictive behavior.

The deal with addictive behavior patterns is tapering off rarely
works.  It's necessary to completely stop, go through whatever
withdrawals happen, then competely avoid the trigger foods
from then on to keep the cravings from coming better.

If you subscribe to the theory that the only diet you can stay
on is one with no forbidden foods, think of how you eat cookies.
Bluntly, any diet that allows you to eat one cookie a day,
exactly how long are you going to be able to stick to that diet?
Sure, the theory may work for some, but the fact that I can walk
through the Mall and see lots of people fatter than me tells it
it's a failed theory for most.

If you have binge eating problems with sugary foods then you
need to go on a low carb diet that teaches you what you can
and can't eat without cravings.  That's called Atkins.  The real
Atkins as described in the book Doctor Atkins New Diet
Revolution is a process that starts with a two week cold
turke purge - It will turn off your cravings though it's not an
easy two weeks.  Then it gradually builds to a mild diet plan
that lets you continue losing in a phase called Ongoing Weight
Loss.  Then once down to a nice weight it moves you on to
Maintenance.  It's a system and a process that if you follow it
will turn off your cravings within two weeks and by the time
you've moved to Maintenance you'll know what foods do and
don't trigger cravings.

The long term problem remains that temptation never, ever,
lets up.  If you take on the view that no food is forbidden then
you'll take the first bite of one of those cookies and you know
how you react to that.  But Atkins does put you in a situation
where you don't have cravings for that first bite.  It works but
it is as unstable as an alcoholic on a 12 step program.  One
drink and the alcoholic starts over.  One cookie and you or I
start over.  But it does work while on the plan.  Try it.