Are you not just doing this as a favour for your brother in law? Unless you have bigger plans to work for paying clients you wouldn't have to register as a MIP to my knowledge. If he is paying you and that's all you're doing then it's not worth the hassle (and cost) - do it for free. You might get a bigger Christmas present next year ;) If you had to register in these circumstances I'd say there's lots of students and members out there breaking the rules by doing family members' tax returns.
