I really can't help in any detail other then some general observations. The photo is clearly either from the second half of the nineteenth century (which would fit with the time your husband's great x3 uncle was in India) or possibly the turn of the century. These are relatively senior officers*, possibly in the Bengal Army. You can tell this from the amount of gold braid or lace on their sleeves and the fact that they are wearing trousers with shoes and not jodhpurs and riding boots. As this is not a formal studio photograph, perhaps it was some great ceremonial occasion such as the Delhi Durbar of 1877 to mark the proclamation of Queen Victoria as Empress of India, or possibly the Durbar of 1903 to mark the accession of Edward VII. I am almost certain the key to identifying the men, and possibly the date, will lie in the turbans they are wearing.
* senior by Indian standards pre-Independence, meaning probably no more than Majors.
