(a) what does the gemora mean by saying that amon & moav take ma'aser oni on shvi'is in order to make sure that the ani'im of eretz yisroel have what to eat? surely on shvi'is all the produce in eretz yisroel is hefker and everything is available to them right in their own back yard!
(b) why do we find that the gemora saya in makos that we need an extra posuk for a full fledged sister and we would not know it from the issur of a half sister yet here we assume from the issur of his wifes daugter to include his own daughter ?
(a) During the Shevi'is year, no staple produce grows in the fields (since it requires planting), except for fruits of the tree, as the verse states, "And if you ask, 'What will we eat during the Shevi'is year, for the fields have not been planted?'" (Vayikra 25:20). (In addition, there is an opinion in Pesachim 50b that says that Sefichim, which is wild produce that grew by itself during Shevi'is, is prohibited mid'Rabanan to be eaten.) Therefore, there needs to be some other source of support for the poor people.
(b) TOSFOS (Yevamos 2b, DH Bito) discusses this point. Tosfos answers that the verse of "Achoso" is different, in that it implies that it is talking only about a sister from one parent and not from both parents, as the verse says, "Moledes Bayis O' Moledes Chutz," which the Targum Yonasan translates to mean, "From the mother or from the father."
In contrast, the verse of "Bito" does not specify where the daughter must come from, but rather it says generally that any "Ervas Ishah u'Bitah" is prohibited.