Real life MMD: Should I ask my brother-in-law for unpaid rent?

depends how long ago it was. from how it's written the implication is that it was some time ago - in which case why have you left it so long? why didn't you say something before she left? why did she move out - was the rent situation the prob or something else? did the room sharer pay anything? to you or to her? a pitfall of renting out to friends or family is that it's harder to clear up probs like this.
if it was a very recent situation then I'd say yeah, maybe you should ask why the rent hasn't been paid but if the bro-in-law has done loads of jobs, above and beyond the call of duty then your wife is kind of right.
if it was quite a while ago then it's a bit mean-spirited to bring it up now unless you're really desperate for cash.
lesson to learn is put it all in writing even if it's family & have clauses for what happens if rent isn't paid. or actually, don't rent to friends/family - let a stranger move in and see if that's any easier!!

A bit more information would be helpful here!

On the face of it, I'd say it's perfectly reasonable to ask for the money.

However, this dilemma brings up so many unanswered questions, it's difficult to know where to start.

What did you agree with your niece and bro-in-law? If he agreed to pay rent, but hasn't, then you should ask for it.

He didn't just waltz into your home and do jobs without you noticing - so what did you discuss at the time? If you agreed to deduct money from his rent accordingly, then do so. If not, don't.

By "shared the room with someone else", do you mean a permanent roommate? If so, this roommate should have had their own separate rental arrangements. Or do you mean she had a boyfriend / friends sometimes staying over? If so, this shouldn't impact on her rent - she's responsible for her guests. But again, what did you actually agree here?

Did your bro-in-law or niece pay a deposit upfront? I'm guessing not - but if they did, use this to cover as much missing rent as possible.

And why didn't you ask for the rent at the end of each month, instead of letting it accumulate until after she'd left?

P.S.
If you ever let a room out again, please let me know. I'd love to stay somewhere where I can go several months without paying rent and then leave, no questions asked.

If your niece and another person rented the room on that basis they should pay, family or not. We would all put our family up, but when it comes to a formal agreement it should remain that as it is stopping you renting to some other person who would actually pay.
If the work can be measure in monetry value then knoock that off the rental amount and ask for that.

In my opinion you should ask for unpaid rent as soon as it is due and follow up immediately if it stays unpaid - not wait until the tenant has long left the property! If you were going to be strictly business about it, you should have let your brother in law know from the off that you expected your rent to be paid regularly and on time, as you were only letting the rooms because you badly needed the income from them. In this respect there seems to be a woeful lack of communication here.

Instead it seems that you let matters slide and now you are feeling a degree of resentment over not having received that money. But it does sound as if you were not clear enough and didn't follow through properly when rent became overdue. So I think it is all way too late now.

When you add in the tricky position of this being your family - and your wife's apparent lack of support for your feelings about it - I really do suggest that you walk away from this and with a better idea of how to handle future business ventures.

I certainly do not recommend sueing your brother in law for the money! That is a very unpleasant route to take and would surely be extremely distressing to everyone involved - not just your wife and yourself but to your niece, your mother and father in law etc. Such things radiate through families, causing rifts and untold unhappiness. Money is not worth that - especially when it was really down to YOU to have set out clear parameters from the start and followed through in a timely manner.

As a rule of thumb though - and with families there should definitely be no exceptions - get financial arrangements put in writing.

Sorry you are out of pocket - but try to get past any sense of injustice or resentment and maybe value the contributions your brother in law made a little more - not just in financial terms but in supportive ones. He not only contributed his skills (and maybe materials and use of his tools) but when he did those odd jobs around your house, he gave his time and care too.

This is the problem of dealing with family. You have to take it on the chin and move on. Rules of money mean that you need to be keen to keep on top & if you don;t it slips away. Hopefully a lesson learnt. If you keep harbouring these thoughts of getting it back it's going to start souring your relationships with your wife and family. If the money owed is actually for board and lodgings - or your utility bills shot through the roof whilst she was there; maybe you need to get your wife on side. If she's not willing - let those sleeping dogs lie.