The first thing I would do in your situation is to discuss this whole thing with your girlfriend. This is an unlikely coincidence, but coincidences do occur. Just think about it, approximately 750 million women living in India, you have a six-month affair with a mature woman, and then unbeknown to you, you meet another girl who turns out to be her niece. If you sit down and talk to your girlfriend, and explain the situation, and that her aunt is trying to recommence the previous liaisons, then at least your girlfriend will know that her aunt is not to be trusted, and that you are.
Alternatively, you can tell this auntie that you do not wish to re-commence the affair with her, and if she even thinks of trying to use your former relationship with her to destroy your relationship with your girlfriend, that you will expose her to all of her family. She has more to lose than you do, as she will not want to suffer the humiliation of being exposed to all of her friends and family as being an attempted blackmailer. You haven't mentioned whether she is married, but if she is, then while you will come in for some criticism for having sex with a married woman, she will be reviled for cheating on her husband. So really, her threat of blackmail is hollow, as she stands to lose far more than you.
There is one ambiguity in this whole situation, and that is what you really wish for? In your last statement, you said that she started to seduce you again, but you did not elaborate. Are we to assume that you finished up in bed with her again and had sex? If this is the case, then I have to ask you about what your real intentions are. If you really love your girlfriend, then resisting the advances of another woman should not be that difficult. But if you finished up having sex with her, it would suggest that the idea of having a girlfriend, and a secret mature mistress may have some appeal to you.
What people choose to do is really up to them, and the morality or otherwise of this situation is not my concern. However, I would have to say that trying to manage a girlfriend who expects a loving relationship, presumably including plenty of sex, and a 40+ woman who probably has a voracious appetite for sex, is both demanding and exhausting, and I guarantee that sooner or later you will get caught. If you really care about your girlfriend, and you suggest that you love her and want to marry her, then any idea that you might have about keeping the auntie satisfied as well should be put right out of your mind.
If you want to marry this girl, and it sounds like you are serious about this, then make sure that she is not in the dark about what happened in the past, or what her aunt is proposing. If you put your cards on the table and tell her the truth, she might be angry or disappointed at first, but at least she'll know she can trust you. Trust in a marriage is essential, if you hope for it to succeed.