My dad lost a lot of weight, 4 stone in 4 months. His appetite had gone and he ate less and less. He had tests but everything was negative so I could not understand it. Then he started vomiting after only a mouthful of food so his GP got him an urgent endoscopy and we found that he had oesophageal cancer and he could no longer physically swallow food.
Dad was 87 and very frail and they would not give him any invasive treatment like an operation or chemo. His oncologist said that he would not withstand either and we agreed but they did give him a stent so that he could eat again and he lived quite happily for another two years. This was a simple procedure that dad was not even aware of as he was sedated.
If your MIL is eating quite well then she is not likely to have the same cancer as my dad which is good. You don't say how old she is but in my experience the medics are reluctant to perform invasive tests on the elderly, others may have different views. Dad had palliative treatment and he was happy with it and to be honest it was very good. They took a watch and wait approach and dad had a better outcome than some of the younger patients who had the full treatment of operation and chemo.
I agree with
@Shedrech that it would be a good idea to get her GP to check her over in case it is something else, he is not likely to do anything invasive to her but he might find something that can be treated simply.
In my dads case he certainly benefitted from knowing and a very simple procedure improved his life and gave him another two years.