[matthewb0051]

Preface: not intended as legal advice and I'm not licensed in Canada.

Assuming this occurred in Canada, you need to get an attorney that is an expert in this stuff in Canada. There are too many variables to consider and possible remedies as well.

Depending on the law there, you may have common law spouse, commingling of assets (assuming there is a community property and separate property law provision there), if common law then you possibly have a right to a spousal elective share for spouses that are written out of a will.
The 1st spouse's pension may have a right of survivorship same for the dad's pension (which given the facts likely is not impacted by this woman's death). That may not allow for mingling it with the paramour's assets. Either way, those typically pass outside of a will and her actions may have no bearing.

Short answer is go see an attorney. Cousin not you. This is not something that will not be easy nor agreed to by beneficiaries without court intervention.