We have told you before , your sisters boyfriend is here illegally. He claimed to be a Biochemist to get a work visa but was working as a security guard, that is illegal , he HAS to work in the job specified in the visa and you can not get a visa to be a security guard they are only highly specialised jobs ( such as a Biochemist)
He HAS to work and it MUST be in the area specified in his visa. This man has deliberately targeted your sister and got her pregnant to use that as a way to stay in the UK. His next move will be to claim asylum in the UK by claiming his 'right to family life' under the human rights act.
At the moment your sister is committing an offence by harbouring him , she needs to throw him out and report him to the police as an illegal immigrant .
Hard I know, but the longer he stays in her house the worse the situation will become.