Shared property is tricky when agreements aren't made up front, but the right answer in a situation like this is basically whatever you agree to. That said, you own 5% of the property, not 5% of what it used to be worth. I would encourage you not to dismiss the value of that equity over all these years. If you had gotten 5% of the value 25 years ago you could have saved/invested it and increased it several-fold.
The three factors to consider are time value of money (all the interest you could have earned if that equity wasn't tied up in the house), contributions each of you have made over the years, and benefit each of you have received over the years (rent, or usage of the property).
The time value of money concept makes 5% of current value the logical starting point, but from there it might make sense to adjust up or down based on contributions made and benefits received.
For 25 years have you been paying 5% of the upkeep/property tax/etc? Have you gotten 5% of the benefit of owning the lake house?
If your brother has gotten 100% of the benefit of owning the property for 25 years, but has also paid 100% of the expenses then 5% of current value could be fair. If you've gotten some benefit out of the house without contributing all these years then maybe less than 5% of current value is fair. If you've contributed to upkeep all this time and not gotten much benefit from owning then 5% of current value could be too low.