dear Riya very weel said. and the thing is that people who are skinny or ve weight, mow they are blesed with a motivating partner who can help them get in shape.
Personally I think curvier/bigger women are so much more beautiful than the average skinny chick. Just look at all the times Renee Zelwegger gained weight for that movie (name totally escaped my mind right now.).. she always looks better a little fuller than she did when she was skinny. My sister gained quite a bit of weight after her baby and I think she looks better this way.
Everyone has their own preferences. And you need to stand up for whatever you want. If you like the girl and are willing to marry her, than go ahead and do so. Your mother and sister arent the ones that need to be attracted to her.