Can a in-law sponsor their spouses grandmother?

Hi, not even a US citizen can sponsor in-laws.

If you refer to a form I-134 affidavit for the purposes of a B-2 visitor visa, then this is possible. However, all applicants for non-immigrant visas are 'presumed' to have immigrant intent, which the applicant must successfully rebut, if challenged, otherwise the B2 non-immigrant visa must be denied. There are other reasons for an applicant's inadmissibility, so make sure that those won't affect her non-immigrant visa application. Good luck.

Sorry, that is not possible. You should consult directly with an immigration attorney to determine if other ways exist that she may be able to come over at least temporarily.

NO you cannot sponsor an in-law. US Citizens can petition for spouses, children, parents and siblings. But not in-laws. If she wants to visit the US she can apply for a visitor Visa.