Are epidurals effective for overweight women?

Yes. Epidurals are effective for overweight women, though, it may sometimes be a little more difficult to place an epidural depending on the amount of adipose tissue present.
Yes. Epidural anesthesia is effective regardless of body weight if the needle can be inserted with the tip in the correct location - in the epidural space. In very overweight patients it may be more difficult to correctly place the needle, so a longer needle may be required, and the patient may be placed in the sitting position which could help make insertion easier by aligning and curving the spine.
Yes. Dosing may be adjusted and it may be a more difficult placement if the patient is morbidly obese. Otherwise, it should work well.