How do I stop tax buyer from claiming deed on family property after redemption date

You really don't, You had all the time the taxes were due. You had the entire time during the redemption period.

After that point the buyer of the taxes normallly just files to get the deed. So you can try and buy the tax certificate ( or what they use in your state) from him, like during the redemption period. That is normally ( but varies by state) but it is the amount he paid, plus 10 to 12 percent interest per year.