The -세요 and -셔요 endings can be used in declarative sentences too, but the restriction is that we do not use them for the first person "I" because doing so would raise oneself specially, which should never be done in any circumstances. (It's similar to judges in court not referring to themselves as "my honor"- the phrase is not intended for the person speaking) Your example's subject is "mom", so it is a natural declarative sentence. When there is no 요 at the end, as in 이쁘셔 or 드셔, 셔 should be pronounced as is. 셔 becoming 세 only happens with 셔요. It is this way because 세요 is smoother to pronounce than 셔요 as it doesn't have the beginning [i] component in both syllables as does 셔요.