You need to find out what people are feeding her on the side. Explain to the family that this is a very serious issue for this breed -- that IF they are feeding her treats on the side, it MUST be calculated in her daily food. Maybe set up a time of day when she gets a treat and then assign a different family member to have this privilage? And shift those treats to healthy things like carrot slices, pepper slices (not hot pepper but green or red or yellow sweet peppers), and apple slices (no seeds; they are poisonous to dogs), pear slices, a strawberry, things like this are much much better for her. The fat on meat should all be trimmed off and you should be the only one deciding if she gets a meat tidbit. Better to save scraps and offer as part of her meal and avoid meat as treats, where people might be sneaking fatty pieces. You could prepare some healthy treats and keep them in a cup of cold water in the fridge so they are always available and make a healthy choice an easy choice, too.
Edel, the amount to feed really depends on what your vet thinks is correct. Lady is already larger than my 2 year old Jaspar, who is about 7kg. She will probably be a largish cavalier anyway so it is hard to guess what she should be getting but Jaspar gets in total about 130 grms or 3/4ths of a cup of food daily. This might be 2-3 raw chicken necks as the total meal, or a single handful of Royal Canin plus half a tin of sardines, or the RC plus half a finely grated large carrot (I have a kind of file grater which is great for this, or you can use the pulverised remains fo fruit/veg from a juicer), or about 1/2 cup cooked chicken or beef plus the veg, maybe a dessert spoon of cottage cheese or yoghurt, etc. I wouldn't be feeding Lady any more than 1 cup TOTAL of food throughout the day and only veg/fruit treats on top of that. These are very small dogs. Half a chicken breast is a lot of meat. That plus the rice is probably already a full day's meal right there. Try measuring it; maybe you should be giving half that amount.
Exercise will definitely help -- a couple good walks daily.
Have a read through this and also have your family read it! The Roycroft page on feeding is a must -- the link is there on this page:
http://www.cavaliertalk.com/phpBB2/viewtopic.php?t=465
Also read through the MVD info here -- this is what your cavalier will almost certainly face *anyway* and you don;t want to bring it on any earlier!! Note Pat, who has owned cavaliers for years, says it is very important to keep your cavalier THIN:
http://www.cavaliertalk.com/phpBB2/viewtopic.php?t=561
Breeder Laura Lang (Roycroft) has noted elsewhere that while her own dogs -- which are bred for heart health -- tend to live til 13-15, most that she places with families as pets live til 11-12, which she links DIRECTLY to the fact that they are overfed. So -- being overweight trims a couple of years off a cavalier's life even if the dog is not prone to early-onset MVD.