Maisarah means “ease, comfort, prosperity, abundance etc.
Ummu Maisarah means the Mother of Maisarah.
We are unaware of the meaning of siti.
We would advise you to make mashwarah with your wife to decide on which one to choose. May Allah make your daughter a means of ease and prosperity for you and the entire Ummah
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai