Visa extension

Her second term extension won't be granted unless she has a strong reason like health treatment.

You can't apply on your own, only PR and citizens can apply visa online. She can try to come back again after few weeks. She can tell that she is visiting her husband who works here. Avoid repeat visits so frequently.

You can apply DP for her once your salary is 6k or above. Good luck

I strongly recommend NOT to (ab-)use visit visa in this way to, in fact, have her live with you in Singapore.
A second extension will not be given without good reason and for a subsequent visa she is at the mercy of ICA, meaning it is luck if and when she can come again. A third visit visa application shortly after that will most probably be rejected, so then she'd have to stay away for many months or a year.
You should use the appropriate residence visa types for this purpose: DP or LTVP.

Thank you beppi and surya for your kind reply.
Sorry for too late reply.
She went back to india yesterday.
Can i apply visa for her using my singpass after 15 days.

Did you read my response? I have mentioned that only a PR or citizen can only apply visa. So, your cousin can apply but should be cautious here. Your wife simply can't come again and again in short span, immigration officer will ask her questions and they have every reason to send her back. So, apply in gap of few weeks to few months (gap should be wider after she visited few times already).

A good rule of thumb is that she should stay out of Singapore for longer than she was in the country before.
(There is no formal rule for this, so every subsequent visa application a bit of a luck thing, but the above seems to work at least for a few months.)