The serial is looks like the serial Gran Hotel, but maybe the period is about +=20 years.
I saw the serial from 2003 – 2008. Mother with two small boys stay alone and return to the city where her mother and sister live. Husband is either dead or gone.
Little sister falls in love with a cheater. Mother was categorically against this marriage. Sister runs away with this tr. When she finds out about her husband, she left him. The he left her the letter and tells that he goes to war (maybe).