At the very least you will want to attach your bank statements. Even though you plan only to provide them with a room, it must be clear for the ECO that your situation in the UK is well and legitimate. Although it seems a bit weird to me that you are willing to sponsor the room but not willing to support them too. Is that something outside of your means to offer? I ask because of V 4.4 of immigration rules:
V 4.4 The third party may be asked to give an undertaking in writing
to be responsible for the applicant’s maintenance and accommodation.
In this case paragraph 35 of Part 1 of these Rules also applies to
Visitors. An applicant will normally be refused where, having been
requested to do so, the applicant fails to provide a valid written
undertaking from a third party to be responsible for their maintenance
and accommodation for the period of any visit.
So it's something you should be ready to offer.
As for proving ties, this is a bit more tricky. To find more details, I will reach for version 5.0 of Visit Guidance, namely the page 15 is what interests us here. There you can read:
To assess whether the relationship is genuine and whether the third
party intends to provide support consider:
- the third party’s previous history of ‘sponsoring’ visitors - for example, previous failures to support visitors may call into question
their intention and ability to do so for this application
- the relationship between the applicant and the third party
And continues with
For visa applications, you may want to check with the applicant or
- where they met
- how often and by what method they communicate
So from the sound of it being able to show that you are close with your brother in law and friend, as you communicate often and have known one another for a substantial length of time is the key. I imagine that in establishing that a long history on Facebook chats, exchanged emails or record of phone calls together will help, but what they are really looking for is previous visits. Do you fly to visit your brother in law and your friend? Or do you travel somewhere together? Including a list of dates, places when you have travelled to visit them, or they have travelled to visit you before, will go a long way establishing that it is a genuine relationship.
Additionally I think that a solid proof can come from the relationship itself. As I was thinking about how would I prove my close ties with a friend I could start by telling the story how we met in an online game 13 years ago. I could then say how we talked outside of the game, met for beers and travelled together across Europe - seeing how many bars we can get kicked out from. How then in recent years I have flown across the globe just to attend his wedding. When you are close to someone, then there are plenty of memories that you share. And when immigration officer asks about the relationship will be the time to share them.
The reason for their visit to the UK is also crucial and it must be clear to the Immigration Officer that it is indeed an important occasion, as without it the application is likely to be refused.