That's a good all-original watch with a scarce and desirable dial variant in apparently pretty decent condition. I think I detect a stain around 2 o'clock but it's still very acceptable and wearable. A regular jeweller will have trouble opening it as this is a one-piece case which opens from the front, so it needs to be seen by a proper watchmaker. Date somewhere around 1963-67. The movement will be the automatic 552 calibre.
It needs servicing if it's going to be worn, otherwise the old dried-up oils will grind and damage the mechanism. Do not sent it to Omega, find an independent watchmaker who will do an equally good job more cheaply and usually more sympathetically. If you are in the UK, such a service should cost around £150-200. You can pay more but there is no particular need for a "gold standard" service on a watch that's worth about £400 as it stands and shouldn't need much doing. It's important to keep the original crown and crystal where possible and not to get the case polished.
Women's watches used to be smaller but the former men's size has become the default women's size, so your auntie seems to have been somewhat ahead of the curve. The Seamaster De Ville was and still is an excellent, understated model with a movement as good as anything else out there at the time. Treasure it.