Crew do make commission from drink like most airlines that sell drinks etc.
I wasn't there so I'm not going to criticise or indeed defend the crews actions.
If they were refusing to sit down for take off then as a No.1 i'd seriously considering asking for them to be offloaded. Occasionally you get someone standing up once the cabin is secured but they sit down when they are told to.
However, it is not always clear cut when it comes to dealing with these 'groups' and on certain routes we do tend to get a lot of them at my airline too. On several occasions I have had passengers question my actions when it comes to serving groups. There are a few things to consider.
1) Groups of lads (and girls) often 'bounce off each other' and are quite often loud. That isn't a reason to not serve them with alcohol. It's good practice to build a rapport with them as it then makes it easier to reign them in if they decide to overstep the mark. Swearing; most crew will tell them to stop swearing but loud behaviour is common place on certain routes and thats just the way it is.
2) Serving alcohol from the bar helps to prevent their own large bottles of tax/duty free spirits etc being cracked open. Invariably these days passengers are warned about drinking their own alcohol and if you withdraw the bar drinks then you will have them drinking their own for sure.
3) Peoples tolerances vary. I am used to loud behaviour on certain flights, particularly in the Summer. It's part and parcel of what I do, I know I have control and these loud flights can be great fun to operate. However, your nervous flyer or those flying away to some quiet resort may feel intimidated so a cabin presence is always required for reassurance but strictly speaking, being loud is not against the rules and it doesn't always mean they've had too much to drink.
It's worth while mentioning a very recent flight I operated with around 15 rowdy squaddies returning back to the UK. They were loud, and sadly 1 or 2 were obnoxious. One guy in particular was drinking his own alcohol and generally being an idiot. A couple of people complained, I had another word with him to then be told to give them a break by a few other passengers. Again, peoples tolerances vary.
4) Withdraw alcohol if/when someone starts misbehaving. Again, misbehaving doesn't generally include loud behaviour. Withdrawing alcohol to quieten down a group doesn't work. However, if someone shows signs of unsafe behaviour then that is where you start getting serious and where withdrawing drink is necassary.
5) The vast majority of onboard incidents caused by drunken behaviour involve the consumption of own alcohol and believe it or not it tends to be individuals/couples who have been nice and pie one minute and the next suddenly turn nasty.
6) Some older couples can drink just as much. On Manchester flights it's not uncommon to serve the little old couple going on holiday 2 bottles of wine washed down with 2 double G&T on the first bar service. But their demographic means they are unlikely to be loud with their mates.
Like I said i don't want to comment too much as I wasn't there. To be honest, if they were refusing to sit down for take off I'd have considered asking the Captain that we return to stand and I would have been weary if we'd have taken them. Loud behaviour at this time of the year is just part and parcel of flying to the med though and I won't withdraw alcohol just on that basis. I will ask them to be quiet and many will quieten down only for them to get loud again. I'm sorry but that is more of a courtesy issue and not a safety infringment but like I said you keep an eye on them in order to make sure things don't get out of hand and you remain in control.
I'm not dismissing your sister in laws account but I just wanted to make a few points because some scenario's aren't always black and white and passengers don't (and aren't expected to) know how best to deal with groups of lads. Ruling with an iron fist rarely works in my opinion.
I'm not a fan of Ryanair and I am dubious about their crew even though there will, as always, be some bloody good crew members there.