We have redone 4 bathrooms in 3 houses in the last few years and my advice is to go to a local counter top store and look at more options. We found a local place that makes vanity tops in several more options and higher quality than the box stores and they add the sink you want and drill the holes. It was cheaper and we got a way nicer top. We did one from HD because it’s what the MIL wanted. We found it was way thinner material and was not as nice my hubby and I hated it.

I had the exact same problem. I picked out the vanity I wanted but did not think about the fact that it wasn't made to sit flush against the wall. I spent a month looking for something else, but in the end we decided to go with something shorter and have it not be against the wall. But we were going from 72" to 60" which is probably not workable in your mom's situation. I did find a lot of vanities that did have flat sides and could work in a corner, if you are willing to have the countertop fabricated. They just need to not have any trim detail on the top or bottom like the one you picked out does. I would just look at the ones without the pre-made tops. Because the pre-made top will still leave you with a slight gap because of the countertop overhang. This is an example: Avanity Modero which is sold everywhere, not just at Lowe's. The cabinet we ended up buying is an Avanity Madison which won't work for you, but I assume it is of similar quality. We are pretty happy with it. The drawers and doors have soft close but I do feel like the glides in my Ikea kitchen drawers are better quality. It is very pretty and I wanted a stained wood so I am really happy with our choice. If I had been willing to go with a painted finish, I think we might have stuck with the 72" Modero.

Cost of this vanity,including sink? I need 100”

I would replace both counters and sinks. The sinks do not match so even if you do (sort of) match the brown counter they will not look like they go together.