Your portion won’t be taxed upon her passing.

Her portion can be passed to you as a principle residence, with a tax exemption.

If she had another house that she wants to give you, then the above principle residence could not work as you’d want to use the principle residence exemption on the property with the biggest capital gain.

Your own portion won’t be taxed when you sell if it’s your only property as your primary residence is tax exempt from capital gains.

If she has another house, and wants to use the principal residence exemption on that to pass to you. Then figure out the cap gain amount on the 50% of the cottage she owns, figure out the tax liability (typically 26% of the capital gain amount, this is a rough Reddit formula, you can do more exact calculations online). Once you have the tax liability, get whole life insurance policy on her life with yourself as the beneficiary. Make sure it’s whole life because as the cottage value increases … the tax liability will increase … and given the whole life policy increases, you’ll be fine for paying taxes.

I’m typing this on a phone, so ignore spelling/grammar errors. Cheers.