Please provide some more detail: what is the primary languages spoken in their family, and which culture do they live in. It is very likely that the names were chosen almost entirely due to their sound, and the best you can go towards `meaning' is `why was this sound pattern familiar in the language as a name'. Jidhin is actually pretty rare as a sound pattern in North India, and nidhin, though possible, also gives me pause. They do not sound quite north Indian to me, but something from the western part of North India, except changed by the influence of a completely different linguistic culture (with jidhin reminding me of words meaning obstinate, and nidhin of the sea and wealth). I am assuming in all this that dh is pronounced as the aspirated version of the first consonant of English word then. If it is retroflex instead, I have to think again.