First of all you must understand that your step-mother and you have committed a sin, so both of you should do sincere repentance and expiation with real regretting and never ever do it again!
Allah the almighty says:
And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah ] and was evil as a way. (4:22)
The situation you describe is understandable, maybe it was your father's fault to marry such a young girl and leave her for so long alone at home. But it's not him who committed a sin so far! But i think it's irresponsible to leave a young wife with a son in the same age "alone" at home! The Sahabi Jabir (May Allah be pleased with him) married a woman who wasn't virgin after his father died just for the purpose that his little sister will be eductaed by a woman which is older then them as we find in both sahih books (al-Bukhari & Muslim) this should be an example to follow!
An interpretation of ibn 'Abass (May Allah be pleased with both of them) of the Verse from surat an-Nisa' (4:22) which is quoted by al-Bayhaqi is that "whom your father married" means: any woman your father married no matter if the marriage was fulfilled (by intercourse) or not is haram for you!
Therefore the option to marry your step-mother is out of reach as it is haram and you should forget about this! It might be the best if you get married with someone else or leave the household of your father. But at first you have to repent your sin and leave it totally.
I'm not that good at giving advice's so take the following with care:
- You should think about informing your step-mother about the committed sin! And tell her to repent and leave it!
- You should refuse to have contacts with your step-mother which go further then the normal relationship mother/step-mother and son (of the husband)!
- You should -if you stay at your father's house- let the contact between you and your step-mother be reduced to the minimum.
- You should think about informing your father if your step-mother didn't care about the sin and still tries to have you near in the way you described!
- Again you should think about leaving your home if you can't stand the situation.
- You should find an other girl and marry!
- You should do your best to forget about what happened between you and your step-mother.
- You could fight these bad thoughts by fasting!
- It was narrated that 'Abdullah said:
"The Messenger of Allah said:
'Whoever among you can afford to get married, let him do so, and
whoever cannot afford it should fast, for it will be a restraint
(Wija) for him."'
- Abdullah (b. Mas'ud) (Allah be pleased with him) reported that
Allah's Messenger (ﷺ) said to us: 0 young men, those among you who can
support a wife should marry, for it restrains eyes (from casting evil
glances) and preserves one from immorality; but he who cannot afford
It should observe fast for it is a means of controlling the sexual
[Sahih al-Bukhari & Muslim]
And Allah knows best!