How do they get around at home? What type of assistance are you looking for? What issues might they have at an amusement park?
I'm a T6 complete paraplegic so my needs are different from someone who is a quadriplegic or an amputee or various other disabilities. However Disney does not provide any assistance inside the parks for me. All of my needs are mobility related so the DAS isn't something that I would need. Disney doesn't push your wheelchair around the park or lift you on and off of rides so I have to do those things myself.
For Disney hotels and transportation Disney does provide some special services. When I make my hotel reservation I make sure I have reserved a roll-in shower room because I can't easily use a tub. When I book my Disney Express to and from the airport I make sure they know I have a wheelchair and will need a lift equipped bus. When I travel between my resort and the parks I use the internal Disney bus transportation and board from the wheelchair boarding area and use the ramp into the bus and the wheelchair tie downs.
No assistance with rides or things- the cast members will accommodate wherever they can, but they will not lift people or chairs, etc. If they can transfer at all, they can ride most rides. If they can’t there are some rides that are ECV and wheelchair accessible.
You said you are thinking of renting an ECV for them. Honest question, why are you thinking that? Do they use more limited mobility devices at home? Would they be renting wheelchairs if not an ECV? Do you have a good understanding of their day to day capabilities? I ask because there is a big gulf between someone who uses a cane or no mobility device in their daily life but needs some more involved help at Disney, and someone who has a more severe disability but has their regular equipment and knows exactly what they do and don’t need. The former is likely to thank you for the help. The latter is likely to rip you apart for trying to “help” by taking over. Do your brother and sister-in-law need hand holding, or would they be better able to research solutions to their own needs?