That's the stupidest revenge she could do.
1) She will be the grandmother of the child, so she would have to take care of it too or abandon them like she did with the MC and prove once again that she is the worst mom ever.
2) MC could end up leaving the house with Sheila to start a family, and then she would lose the dick she is lusting so much for.
Also, where was it even mentioned that Sheila is taking contraception? I can't, for the love of all that is porn, remember one single instance where she confirms she is taking the pill, but that could be a memory problem on my part. If you can provide a screenshot with a line like that, then I give you this one.
But, well... considering how stupid Tomoe is... that would actually fit her.