All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad
is His slave and Messenger.
Sterilizing cats for an overriding beneficial purpose is permissible. It is stated in Al-Muḥīṭ Al-Burhānī: "It is mentioned in the chapter on Uḍḥiyyah of Al-Nawāzil, concerning the castration of cats, that there is no harm in it if it serves a benefit or wards off harm from it." (End of citation)
Accordingly, it is permissible for you to sterilize these cats, whether before they give birth or after. Likewise, aborting the fetuses of animals is permissible if there is an overriding beneficial purpose for doing so, and this is not comparable to the abortion of a human fetus.
Allah Knows best.