My mother passed away 10 years ago. Her brother has just died, and there are two heirs (my sister and I). Will we benefit from the tax allowance that my mother could have claimed if she had survived her brother?

No. Representation in matters of succession is a legal fiction allowing the heirs of a person to take their place in an estate because they are not present (due to renunciation or death). For this mechanism to apply, there must be a plurality of branches (in practice, your mother would have needed to have other brothers and/or sisters) (Article 752-2 of the French Civil Code).
In your situation, the mechanism of representation does not apply. You and your sister inherit in your own right. Each of you will benefit from a tax allowance of €7,967 corresponding to your degree of kinship (uncle/nephew) (Article 779 V of the French General Tax Code).