What else do you think the government can legally hold OP financially liable for? Here is the definition from IRPA:
https://laws-lois.justice.gc.ca/eng/regulations/sor-2002-227/page-30.html#docCont
Undertaking — duration
132 (1) Subject to subsection (2), the sponsor’s undertaking obliges the sponsor to reimburse Her Majesty in right of Canada or a province for every benefit provided as social assistance to or on behalf of the sponsored foreign national and their family members during the period
The government can come after OP for welfare repayments. If the mother-in-law cannot afford a nursing home bed or dental or whatever else you have referred to and OP refuses to pay for it, she will go on welfare to pay for it. OP would then be liable for the welfare.
In this case, she has enough income that she won't likely won't ever qualify for welfare even if OP refuses to pay.
If she doesn’t qualify for welfare and refused to go on welfare then it would go back to the province looking for payment for the bed. There was an example on this forum of the nursing home arranging payment with the sponsor based on their income. The SIL had not realized this could be a cost. Someone was asking why they would be charging them a fee. Most people are unaware that nursing homes are highly subsidized but there is still a bed fee. Not going to search the forum. Never said dental. It is an expense to consider if parents are moving Canada but it would always be and private insurance provides such bad coverage that it is pointless.