Sure Purva did a lot for her sisters - she went ahead and sacrificed her own love to marry her sisters husband to save him from calamity.
But Smriti - Gets no love from her mother in law, her MIL drives her crazy (actually), she goes to live at an asylum, she's pretty out of it even to properly realize her parents have died, she meets with a horrific accident, her sister marries her husband while she is alive, she is kept away from her own son, FINALLY - she comes back and finds out the truth about her husband and sister's relation.
And now, Mihir acts like a jack, getting so affectionate with his second spouse so fast after Smriti's death. This, is actually very unrealistic knowing how much Mihir loved Smriti (a creative flaw).
But I think I know where this serial is going. Mihir will be showing affection for Purva. Mihir and Smriti will eventually get together as Mihir will realize Smriti's alive and all that she been through. Purva will most likely be killed off the show in the end OR fall in love with someone else. Because serials make it so easy and quick to fall in love with someone so deeply. It's very predictable...we've seen it a million times.