Yet another question about incest.
I am not interested in moralising or judging you, but as has been pointed out, in giving your mum sleeping pills, and then performing sex acts on her, you are committing sexual assault, and if she so chose, she could have instigated your arrest. Have you considered how that would have affected your marriage?
If what you are telling us is true, and I don't see that you would have much of a problem if you spoke to your mum and told her how you feel. You say that she has deliberately put on her bra and panties after finding them coated with your semen, so this suggests to me that the idea of having sex with you would not be repugnant. Many widows find that their need for sex is not fulfilled simply by masturbation, and it is clear enough from the things that we see on this forum, that some widows choose a close relative with which to fulfil their sexual needs. Very often the widow chooses to sleep with her brother, or her son.
You have to consider what effect this would have on your marriage, if you did start having sex with your mum. Unless you were both very careful, your wife would eventually discover what the two of you are doing, and how that would affect your marriage, well, you know better than me about how your wife would take it.
I always tell people to think of the consequences of their actions, and all too frequently, they ignore my advice, with disastrous results. Supposing you do start having sex with your mum: how long would you continue to do this? What would happen if you get caught? What would happen if you got your mum pregnant? Unless she has gone through menopause, there is a distinct possibility that she could become pregnant. You could wear a condom, but what would you do if it broke? She could take birth control pills, but these are only 99% effective, supposing she is the one percent of women that get pregnant despite taking birth control measures? How would it affect your entire family if having sex with your mum became common knowledge to everyone? These are the consequences you have to think of before you engage in any kind of activity with your mum, assuming that she is enthusiastic to do so. If you weigh up all of these consequences, and still wish to go ahead, then I suggest you talk to her discreetly, when the two of you are in the house when your wife is away.
There is always the possibility that your wife would be in agreement with you having sex with your mum. She may even find it exciting, as many people get a thrill from something which society frowns upon. Breaking taboos can be extremely exciting, and while incest is not illegal between consenting adults in India, breaking the social convention can certainly be exciting for some people. You haven't told us whether your wife knows of your secret desires, but if she does, perhaps the thought of it turns her on? But probably not. Most women don't want their husband to be having sex with anyone else, and will do everything in their power to prevent it from happening.
What you want and what you can have in this world are not always the same things. If you weigh everything up, I still think it's worth going for it, then talk to your mum and see what she thinks.