@EnidSpyton
OP have since seen your other posts about your sister's behaviour changing since she's been with this guy.
A lot of evangelical Christian men are very controlling. They're attracted to the church because it gives them legitimised power over women.
Evangelical Christians believe men are superior to women, that they have the role of the head of the household, that they are the authority over the household, etc etc etc. They believe that a woman's job is to 'submit' to her husband.
Evangelical Christian readings of the Bible are built around perpetuating a narrative of rightful male dominance over women. There's also a HUGE and incredibly unhealthy focus on sexual purity and homophobia.
Evangelical Christianity IS a cult. It's led by a small group of men who enjoy the power it gives them over others. Honestly, unless you've been in that world, you will never truly understand how toxic it is.
I would be doing whatever I could to get her out of this relationship before you lose her altogether.
Just in case any of you are really interested in what most "evangelical Christians" believe about men, women & marriage, can I recommend that you go to the source of those beliefs, the Bible.
Genesis 2 vs 18 - 25 is a good starting point and then Ephesians 5 vs 21 - 33. You won't find anything there about men being encouraged to be "controlling" or regard themselves as "superior" to their spouse. Yes, there is some clear teaching on exactly which partner in the relationship God regards as more accountable to Him for the well-being of that relationship - The man. Yes, there's talk of wives respecting and submitting to their husbands - but the corollary of that is an expectation that husbands will love their wives as they do their own bodies. Therein lies the balance. If husbands don't show that they love their wives that much in every decision they make & every interaction that they have with their wife, they are in the wrong as far as God is concerned - PERIOD.
If the prospective partner knows his bible, he'll be aware of the instruction to Christians in 2 Corinthians 6 v 14 not to be "unequally yoked with unbelievers". God knows that a marriage is unlikely to succeed if the 2 partners do not share a like mind on matters of faith & beliefs - or of unbelief, for that matter. By encouraging your sister to attend the Alpha course, he's testing the water to see if there is any likelihood of her being of like mind with him going forward.
In my view, you should be pleased that he is taking this step before he commits to a possible longer term relationship. If your sister baulks at what she hears at Alpha, it is very likely that the outcome will be a parting of their ways, not a veiled attempt to brain-wash her into submission.