Egypt is a Muslim country. The culture and dress code are not as strict in Egypt as they are in some Muslim countries, but it is still best to be modest, especially for ladies.
The dress code is much more relaxed on the boat than it is in the towns. Dress as though you were in a tourist hotel. Shorts, swimming costumes, bikinis are all fine, but have something handy to cover up for when the boat gets to a town or when visiting cultural sites.
Luxor/Aswan can be hot. Even in the winter, temperatures are generally high during the day so you will need light cottons at any time of the year. In some months, around October - March, it can get cooler in the evening and overnight. So for these occasions, take a fleece or a jumper as well.
A basic wardrobe would be loose cotton or linen trousers and/or a longish skirt, 3/4 length pants are also acceptable, cotton tops with sleeves. Clothing, especially blouses and skirts, should not be transparent.
For something a little different, a cotton sarong is very versatile. They are cooler than trousers and you can adjust the length to suit where you are or what you are doing. You can wear it short for the hotel or boat, mid-calf when in Luxor/Aswan itself and to your ankles for mosques and temples.
It is useful to carry a pashmina, chiffon scarf or shawl to help with covering up when you go to sensitive places, such as temples or mosques; to protect yourself from the sun, or if you find yourself amongst a group of 'admiring' men.
You will need 'practical' shoes. The roads and pavements are not tremendous (but improving in places) and to get to many of the sights you have to trample over unmade ground. Comfortable shoes or trainers with sturdy soles are best. Sandals are also fine.