As the title says I would like to know which alignment method(global or local) should I use for sequences for which is assumed that they have common ancestry?

The sequences are 21 aa long peptide chains (proteins).

About the background of the sequences, it's only stated that it's ASSUMED that they are from the same ancestry.

The thing that we assume that they are from the same ancestry makes me confused. I know that on similar sequences with the same length usually, we use global.