The Portuguese word for grandmother is "avó". However, you might want to adress your mother in law in a more colloquial way, similar to the English "granny"; in this case, use "vovó", "vó", or "vozinha".
The formal translation of "grandmother" is "avo. " There are other terms of affection used by different families. But for starters, "Avo" would likely please your mother-in-law.
