So you say the 'd' in
Devika sounds exactly like the 't' in
Sitara, and both sound like English 'th'? I don't think so.
The 'd' in
Devika is a voiced dental plosive (quite like an English 'd'). For English speakers 'th' represents a dental fricative. The 't' in
Sitara is an unvoiced dental plosive.
Tamanna is Arabic, and the final 'a' is long by the way