Is it permissible for my husband to divorce his wife and me, then marry my sister who has not given birth so that she may bear him children, then he divorves her and I return to my husband and she returns to her husband?

Adoption is forbidden because it leads to the mixing of lineages, the loss of rights, and the occurrence of prohibited acts such as khalwah (unlawful seclusion) and impermissible mahramiyyah (kinship that forbids marriage). This applies if the intention is to attribute the child to the one who adopted him. However, sponsoring an orphan or a foundling is something encouraged and desired by the Shariah.

Regarding the question, if the marriage is conditioned upon its continuation until the wife gives birth, after which the husband is compelled to divorce her so she can return to her previous husband, then this is a mut'ah marriage (temporary marriage) which is unanimously forbidden. If the marriage is without the condition of divorce at a specific time, but with the intention of divorcing after childbirth, then this is a matter of scholarly disagreement, and the majority of scholars permit it.