Sona agreed for fasting before Ish putforth the great abstinence clause. Sona really had no other choice than to stick to her decision. If she said no, it wud show her desperation which no one wants to show to their inlaws or parents. N if she said that she wud consult with Dev after she had already agreed, again it wud show that Dev might be getting desperate. There was no way she could avoid any embarrassment.
After marriage we all do lot of adjustments n one of them is started observing many fasts n rituals of inlaws side as well. Initially few years, we all try to give it a try, bcoz we genuinely feel part of the family or we want to please inlaws, or we want to carry on the custom. Some continue to do it later also whereas some leave after few yrs. I don't see any issue with Sona agreeing for the fast. She didn't knew abt Ish's clause, otherwise maybe she wudnt have agreed so eagerly.