As amusing as some of the comebacks are, if the OP is anything like me she won't feel able to stoop to her level and be rude back (especially with family members where it might cause tension).
I would imagine your weight loss is showing and so she's deflecting attention away from your success and also away from her need to lose weight (or maybe your DH is proudly telling her when you're losing and she doesn't like it?!). Be confident and strong in knowing that, regardless of what anybody else thinks, says or does, you will just doggedly keep your head down and keep up with the weight loss. Keep chipping away, a pound or so a week (and time off for good behaviour-birthday/holiday etc), and this time next year you'll be near or where you want to be and she'll be wishing she'd started to as she'll still have her mountain to climb.
As a former Weightwatchers leader, and ex long term smoker, I've found that when you publicise losing weight/giving up smoking it makes people with issues themselves uncomfortable and sometimes they lash out-everytime I announced it at work in the past I'd suddenly have people offering me food/fags ("you've done really well/reward yourself, you've got to have little treats/one a day won't hurt.."). When I turned it around another time and said I didn't want to have cake as the doctor told me to reduce my cholesterol, I got lots of support and no attempts to sabotage!
18lbs off will have already had significant impact on your heart, lungs and joints. Even if you or others can't necessarily see a difference yet, inside you're already prolonging your life. And what is often the case is that people lose a stone or two and others don't necessarily notice, then an extra pound or two off suddenly becomes the tipping point and everybody notices.
Keep at it, and don't let the b&£#^%*s get you down x