I am looking at getting a tattoo with my best friend ( both female) Have been looking at words to describe friendship/ sisterhood/ soul mates. Ideally a sentence or phrase. For example 'Not sisters by blood but sisters by heart" Is anybody able to translate this correctly or something similar as detailed above into latin Any other suggestions welcome
Non cognatione, sed amore germanae.
This means literally "Sisters not by blood, but by affection".
Note that I've used the word germanae (cf. Spanish hermanas) instead of sorores (cognate of "sisters"). The difference is that germana (germanus for a man) specifically means "daughter (son) of the same parents", whereas soror (frater for a man) was also used for cousins and close friends. This means you may well go for sorores if you like it better, but the choice of germanae is stronger for this very distinction.
For instance Cicero in his Rhetorica described his cousin as frater noster, cognatione patruelis, amore germanus, "cousin by blood, brother by heart".
A direct translation of “Not sisters by blood but sisters by heart” would be: Sorores non quidem sanguine, sed corde. That is actually somewhat conversational. For a tattoo you might prefer a pithier version:
Non sanguine, sed corde sorores.
In the meantime, Vincenzo has written an answer that uses the same structure, but different words. My variant uses a rather literal translation: sanguis is blood, cor is heart. It strikes me a bit more earthy, but admittedly, when it comes to idiomatic Latin, you are on the safer side with Cicero …
Other than that, judging by Google Image Search, tattoos saying Sorores in aeternum (“Sisters forever”) seem to be quite popular.