Okay this might get confusing…but my sister-in-law’s sister-in-law, has a son with a name we [name]LOVE[/name]. His name is [name]MALACHI[/name]. Would it be weird if we used [name]Malachi[/name] as a middle name?
I think it would depend on how close you are to your [name]SIL[/name]'s [name]SIL[/name]. If you all hang out on a regular basis, then, yes, it would be too close, especially if they are close in age. If the first [name]Malachi[/name] is quite a bit older than your baby to be, it might be doable anyhow. If this [name]Malachi[/name] lives in another city or even in your city, but you hardly ever see them or hang out with that family, I don’t see a problem with using it.
Not at all. The name is so distant and it’s only a middle name. You’re over thinking it! 
It really depends on how close you are in the physical & emotional sense with this family…on one hand they are your in-laws in-laws which is pretty far removed so last names are different & they probably won’t play together often…on the other hand [name]Malachi[/name] is so distinct…it isn’t [name]Michael[/name] or something.
Age is a factor too. If [name]Malachi[/name] is older than 5 I think it’s really no big deal.
If you know [name]Malachi[/name]'s parents you might mention it to them that you are considering it.
[name]Makai[/name] or [name]Maksim[/name] might appeal to you if you decide against it.
Ohhh as a middle? No big deal at all! Go for it!. I thought you meant as a fn!!
If you’re not going to be calling him the middle, I don’t think it would matter even if it was your own sister. If you are going to be calling him by the middle, it depends on how often you see her, since she isn’t a close relation and I’d be surprised if your children were together much.
You only have a very loose connection to this woman, and even then she does not own the name [name]Malachi[/name]. You may use it if you wish.
That is a distant connection, I would use [name]Malachi[/name] as either a first or a middle name.
I wouldn’t worry about a middle name.
If it’s a middle name, I don’t see why it should matter. Go for it!