Ṛta is a Vedic concept which was overshadowed by the development of the word “dharma”, which is why it isn’t in the EBTs, although you can find some related words.
Utu Utu (m. & nt.) [Vedic ṛtu special or proper time, with adj. ṛta straight, right, rite, ṛti manner to Lat. ars “art”, Gr. damar(t), further Lat. rītus (rite), Ags. rīm number; of *ar to fit in, adjust etc. q. v. under appeti]
…The Pāli word sacca might also be related to ṛta, but showing it is slightly beyond the scope of the time I have available. And Avestan…is not within my skillsets.
[Take 2 as in my previous deleted post I misunderstood Bhante Sujato’s post]
Ved. ṛtá- is to be connected with an IE root *h2er- meaning ‘to fit’.
Ved. ártha- is most likely connected with an IE root *h1er - meaning ‘to reach’