I'm overweight too and I've actually lost some weight since finding out I was pregnant not necessarily because of morning sickness, but because I'm eating less and being more mindful of what I do eat. I'm horribly nauseous a lot of the time, but not actually sick. It helps to eat much smaller amounts a little more often. I find I eat less overall if I never really feel hungry. As simple as an apple in the afternoon gets me by between meals now.
As we cannot exercise as before be more careful with your diet instead. Make vegetable soups get fruits for during the day(dont overconsume bananas). Animal protein like meat chicken or fish but properly cooked.. not fried to avoid the fat. Make shakes with whey protein. Just add some fruits and skimmed milk. Find alternatives for sugar that tastes good like xilitol for those times you might want sth with sugar. Avoid too many carbs but u have to still getting some. Go for natural carbs. Remember your pregnant and the baby needs carbs as well. Look for veggies and salad recipes that u really enjoy because if not you will give up with your healthy diet. Do not forget to get your prenatal vitamins.