Radha may have told tripti her motive it that of revenge and it was probably true until hearing about ahem getting remarried then she was dreaming to become a modi bahu a chance she wouldn't want to give up. Had Radha married ahem the revenge would have fizzed out and she would have done everything she could to remain as modi bahu and would have told tripti she comfortable in life so why wreak it.
Tripti instigated Radha hoping she would get revenge on her and the modi's and something more in the bargain. I doubt tripi realised what Radha would stoop to as now Radha is gloating and happy with what's she's pull off with meera incident. Why she had to confide in tripti I don't know but I guess she couldn't keep this achievement to herself. She has now given tripti something to hang Radha with if she double crosses her.