According to most accounts by Etruscologists, Larthi is the feminine of masculine praenomen Larth. Likewise, Arnthi is the feminine of masculine praenomen Arnth. We may be easily deceived into thinking that the names are pretty straightforward. They're not.
TLE 179: Eca mutna Arnθal Vipinanas, Śeθreśla.
"This (is) the sarcophagus of Arnth Vipinana, (descendant) of Sethre."
The above inscription shows us a male name, Arnth, declined in the genitive (Arnθ-al "of Arnth"). His family name Vipinana is also declined in the genitive as well (Vipinana-s) because this is traditional for family names of males. Depending on the class of the noun, the genitive is marked in either -as or -al. A third name at the end is declined with a case suffix -śla to indicate ancestral lineage. In this case, he is the descendent of Sethre, a common male praenomen in Etruria.
TLE 233: Arnθ Lenies, Larθial clan, Velus-um nefiś.
"Arnth Lenie, Larth's son, and Vel's grandson."
The name Arnth is here unmarked because it's the nominative case and this person's last name is declined in the genitive again because the person is male. There is also no question that he is male because his inscription is written over a picture of him reclining with his brother while being serenaded by musicians. This is what a male name looks like in Etruscan. However if you scroll down through the same inscription, we notice another interesting sentence describing his brother:
Vel Lenies, Arnθial ruva, Larθial[usla] clan, Velus-um nefś.
"Vel Lenie, Arnth's brother, Larth's son, and Vel's grandson."
Notice that the genitive of Arnth may be Arnθal (as in TLE 179) or Arnθial (as in this last inscription). It's not a typo because it occurs elsewhere (ETP 83: Mi Arnθial Tetnies śuθi-θi Velcl-θi. = "I am of Arnth Tetnie in the tomb in (the city of) Vulci."; TLE 320: Ramθa Viśnai, Arnθeal Tetnies puia. = "Ramtha Visna, Arnth Tetnie's wife."). This interloping -i- or -e- is clearly supposed to be there and not just some scribal error. Now let's look at the female praenomen:
TLE 888: Metli Arnθi puia amce Spitus Larθal. Svalce avil LXIIII. Ci clenar acnanas, arce.
"Arnthi Metli was the wife of Larth Spitu. (She) lived 64 years. Having brought forth three sons, (she) raised (them)."
This shows the unmarked female praenomen in the nominative case and we know this is a female name by the context. Note that her family name is declined not in the genitive but in the locative case (Metli < *Meteli-i) as is traditional for female names, while the name of her husband, Larth Spitu, shows yet again the genitivized family name (Spitu-s) as expected for males.
But wait a minute. I have questions and I need answers. Quite frankly I'm not even sure what the answers should be but apparently Etruscologists aren't talking about it either and they should:
1. If the genitive of male Arnθ can be Arnθial, then what is the genitive of feminine Arnθi?
2. Is the genitive of both the male and female names homophonous, and thus only distinguishable by the declension of their last name?
3. If an inscription only says Mi Aranθial (see ETP 295), what gender is the person? (Mi simply means "I (am)".)
4. What on earth is that interloping -i- or -e- doing there?
5. Why is the genitive in -al in the first place?
6. What morphological reasons cause speakers to chose l-genitive over an s-genitive? We have Arnth with genitive Arnθal/Arnθeal/Arnθial and yet Minrva (Minrva-s "of Minerva") and Uni (Uni-al "of Uni").