Hi Concerned Brother,
It is nice of you to be so concerned, and to want to learn more. So please don’t take this the wrong way, but your sister needs to speak to her own doctors about how to address the headaches, and not rely on information a relative gets from a forum.
The reason is that we cannot possibly know your sister’s detailed medical history. We cannot know for sure that her headaches are definitely caused by MS (many neurologists would say this is unlikely, and ought to be investigated for other causes), we cannot know if there’s any reason (apart from the stomach ulcers) she should not take certain things - we cannot know if anything we suggest might adversely interact with anything she’s already on.
Really, she needs to speak to her doctor - who will be well aware of the limitations of certain types of painkillers for a patient who has stomach ulcers, and may be able to prescribe something that is kinder on the stomach, or a stomach protector to make any damage less likely.
They may also want to look into the causes of these headaches, and whether it’s definitely MS. Ironically, repeated use of some painkillers itself leads to more headaches, so it could be the very things your sister is taking for them that are actually making them worse.
Nobody on a forum can know the answer to all these “maybes”, which is why it’s so important a doctor looks at it. The doctor should be taking a much more proactive role than just saying: “Be careful what you take, as you have stomach ulcers.”
They should be looking into WHY your sister has non-stop headaches (this is not considered a “normal” part of MS), and what she can safely do about it.
If you can, encourage her to see the doctor about it (I’m assuming your sister is an adult, not a child?) But also remember that her MS is her disease. What she does about it is her decision, whether or not you always agree. It is she who has to live with her choices, not you. It’s quite easy, for example, for someone who will not have to live with them, to say: “Maybe she should have accepted the risks and side effects?” Only she can make that choice. One person’s “acceptable risk” may be another’s: “Will not touch it!” - we’re not all the same.