I haven't read the replies. but had to get my 2 cents worth in.
There is absolutely NOTHING WRONG with showing your bump in a bikini. I did it with both my bubs (wasn't even wearing boardies. Hate them, find them really uncomfortable when wet) and I'm not a size 8 either.
When not swimming, I often used to wear tight singlets with a belly band covering the gap. I see nothing wrong with showing the shape of your belly. I didn't want it poking out, but I also didn't feel the need to cover it with flowing tops (although I had some).
Also, when it comes to Maternity swim wear and covering the shape of your tummy, when fabric is wet, it clings. So you'll still be showing off your bump (not the skin, though). I didn't see the need to buy a lot of maternity clothes. Especially not swimwear. Although I could have afforded it. That was not the issue.

My mum is older than yours and I have seen many pictures of her in her bikini when she was preggers with me (1979 that was). Granted, that was in Europe. No idea what it was like in Australia.

Sorry, I can't help you understand your mum POV as I don't get it either. Show off that beautiful bump!