My son didn't potty train until three. My daughter is fully potty trained as a young two year old. We bought her really cute panties that she loves, put the potty in front of the tv and let her sit there and watch tv naked while sitting on the potty for the first day or so, until she had some successes and understood what we wanted her to do. and when she did it, we made a REALLY BIG DEAL out of it! :) SO HAPPY! SUCH A BIG GIRL! WOW!!! Also, we cut all fun snacks and candy out of her diet, except when she went potty, then she got to pick one candy out of the jar -- and had LOTS of fun candies in there. Whenever she wanted a candy, we reminded her that she got one when she went potty, so she'd want to go sit right then and usually managed to go! :) Now, my five year old son was a big help through all this, he made her think going potty was the coolest thing ever (because he got a candy, too, when she went successfully, haha). Now I know you don't have a five year old boy, but maybe someday, your daughter can be a big help to a younger sibling, too. :) Alternately, I guess I would say that everyone being supportive of her efforts is important, so if you can get everyone on board with praising her and encouraging her, that's good. So, I know your in-laws don't want her naked or having accidents, but uh, that's just kids! It's just pee. It's sterile. Be respectful and clean it up right when it happens with good cleaner, and without making your daughter feel BAD about it... if it happens on the carpet, it's a GREAT idea to have a steamer if you can borrow or rent one or buy one. (We have a Hoover steam vac, $200 from Best Buy, probably can get it cheaper at Kohl's nowadays!) Totally worth it. We use it constantly.
Oh, also, my daughter went potty like every ten minutes once she figured out what she was supposed to do and that she got candy! So, be prepared for it to rule your life for at least a week or so. And then, once it loses its excitement, for her to have a few accidents while she's busy playing and forgets to go to the potty. And be prepared to carry a couple of extra changes of clothing and some clorox/wet wipes for accidents while out shopping, etc... accidents WILL happen. You'll be taking her to the public potty whenever you go out for a while, until you can trust her to "hold it." It's kind of a mindset change, not relying on diapers and remembering that you have to ask her/take her to the potty every 45-60 minutes, or whatever works for your daughter. Girls usually catch on faster than boys, for some reason. But if she doesn't, don't stress about it. Stop if it becomes "stressful" for your daughter and try again in a month. :)
Good luck!