It's incredibly hard to get DLA, and an EHCP that is worth It's salt.
I'll give you the benefit of the doubt and let's say your sister has managed to get the head on side to say they can't meet need.
They still need to evidence this, usually through the assess-plan-do process or something similar. This is usually done over months. At this point they usually involve multiple other agencies, depending on need. This might be SALT, SCI, even the health visitors or another team which can weigh in on that child's particular need to observe over a few sessions and make professional recommendations for ways in which adaptations can be made to accommodate that child. The school would need to then evidence that even with these changes they still can't meet need.
Then they/she/he/whoever applies for the EHCP which will include an observation and meeting with an educational psychologist who will document the child's difficulties to identify an area of primary need as well as possible secondary needs. Sometimes this meeting is also held as a multi disciplinary meeting with the educational and health care professionals that have been involved. You can't get one over on multiple people who have made their own observations.
If a child does get a diagnosis before an EHCP, that also involves a lot of multi disciplinary involvement and observations usually over a long period of time as these are diagnoses of exclusion. May vary if going the private route.
Then you get you either get an EHCP or you don't and you appeal, then you either get your EHCP or it gets rejected again. If you get your EHCP and it doesn't hold up legally or have the right provision, you appeal again to the SENDIST tribunal where you have the burden of proof for your childs needs and are responsible for organising your own representation or self representing which means you have to become extremely knowledgeable about the appropriate SEND law which means hours of self research and speaking with advocacy services. You don't get to just say I say so and so it is true.
Then to get DLA you have to evidence the difficulties your child has and how they're different from peers in their typical developmental stage. You can't just say something is true, you've got to have paperwork.
There are some things that you just don't have a substantial amount of paperwork for but you can usually verify you've had that teams involvement, and for those you have to meticulously diarise basically all of your childs needs and bodily functions dated and time stamped and the impact this has had on the child and why they can't do this independently.
I've gotten full PIP with less paperwork than it's taken to get my child an EHCP or DLA and getting full PIP is bloody difficult.
Some people would think my little boy also just doesn't like being told no, and that's fine. I've got reems of evidence to the contrary.
Perhaps your sister does actually know what she's looking for, and why shouldn't she want to claim DLA if she feels she is entitled to it due to her childs needs? If she doesn't have the evidence she won't get it, and she certainly can't manufacture evidence when a lot of evidence comes from professionals making their own professional observations.