Please go to this wedding; as others on here have said, it is important to show your DD that you will be there for her if she needs to get out of this marriage and/or indeed, the country.
If she has embraced the culture, has done her research and is happy with this set-up, then that is her choice and if she has a wonderful life that she enjoys then good for her. But while she might be fine, there are many women in such marriages who are living in fear and misery, many of which thought it was all good before they were married.
The thing is that it's not always as it seems and this arrangement with polygamy sounds like it leans more towards the traditional/Saudi type of lifestyle, which would be very concerning. Check out the numerous cases of girls/women trying to escape the country and you will see what I mean. Check out the similar stories regarding women/girls trying to escape Dubai and their families there - especially the more wealthy families. Wealth and luxury does not mean that a woman avoids misery, prison, torture or being murdered on the whim of her husband.
It seems very much that if, as a wife, you do as you are told and don't rock the boat, then yes, you can have a nice and quite luxurious lifestyle. But if you don't do as you are told, don't follow the rules and/or your husband decides he simply doesn't like you anymore... things can go very badly very quickly. I would be extremely wary of bringing children into the mix and your DD really needs to think carefully about what might happen if things go pear-shaped - with or without children.
Your DD must make sure she has separate financial arrangements of her own and ensure she has sufficient independence and freedom so that she can get out if she needs to. It is not just a case of ending the marriage if it's not working out... there is a lot more to it... not least the "honour" element which is where things can get scary.
In addition - where there are multiple wives, it's not always the case that the previous wife is happy with the next one... they just don't have a choice and while some may well be okay with it, many of them just do as they are told - which is - accept it and don't kick up a fuss to avoid the consequences of resisting the arrangement.
I am usually of the mindset that if someone is an adult then they make their own decisions and it is not for you to have any say; however, in this case, the key is for there to be an escape plan in place IF it becomes necessary and for your DD to have means and ways to escape and contact details for the relevant authorities if she needs to get out of there. If it goes well for her, great, but too many women in these countries find themselves in extremely dangerous situations - and the danger cannot be overstated.