Short answer: you need to have a formal, written, signed agreement in place before accepting the job. Even with family.
Also: mixing family with business can be a minefield.
Some questions to ask yourself:
- How happy are you to mix family with business. Family can sometimes increase/change demands and product requirement (including asking you to reshoot or additional shoot) and then say "but you already agreed to do it", "it's just another dress which will look better" etc. How do you place limits on your commitment?
- Decide who will legally own the images, what purposes they can be used for (e.g. personal or commercial), and for what time limits (if any). If it were me I would retain copyright and give the sister in law a licence to use them for x and y purposes and (optional) for z time period. Imagine if you take some stunning shots and your sister in law says you can't use them or show them because they're legally hers. Would you be happy with that?
- Are you likely to be fairly compensated (paid) for your work? If it's a personal favour then perhaps reduced or even no compensation (if no compensation, then what about out of pocket expenses). Depends on your relationship. But what if she then (perhaps later) uses the photos commercially to promote a pageant or her daughter's career. Are you comfortable with that? Would you feel used? That's why most agreements have specified uses and time limits, as well as stating who owns copyright.
- How much time, realistically, are you willing to devote to it? See above about changed demands. Remember shooting time is only a small part of total time taken when you consider travel and processing/scanning/post processing. Spend half a day shooting and several days in image processing. Deliver finished result to sister in law who rings and says, "can you just make them all look more like this (attaches sample from some other photographer from the web)".
- What do you want out of it? Maybe the personal satisfaction and challenge will outweigh all other factors. Maybe the family relationship outweighs all other factors. Do you want the opportunity to make great photos - and also to have ownership of them so you can show them or use them to promote your photography?
- Will this set a precedent that will cause you grief in the future? Will other relatives ask for similar favours at the same rate/conditions? Families, where would we be without them..
I'm very reluctant to mix family with business - in my experience there's too much potential for misunderstanding, exploitation and tears. So I would consider this as either a business proposition, or a gift, and I'd be very careful to ensure a clear mutual understanding of the limits of my commitment and the ownership of my work.
Your decision. At the very least, go in with your eyes wide open.
Cheers,