Bachelor Party & Bachelorette Party are similar indeed (think you understand what they are)
Bridal Showers are meant to “shower” the Bride with Gifts for her new life… and traditionally have included a lot of household items with stuff for the Kitchen, Bathroom, Bedroom (Kitchen Gadgets, Small Appliances, Recipe Books, Towels, Linens etc)
Sexist… nah, I don`t feel that way… just a lot of fun for women to have.
Couples Showers work too. They tend to be more popular with people the couples age than older folks. And often held for couples who have lived together for a long time and don`t need a lot of household stuff.
Themes for Couples Showers tend to be for things that BOTH the Bride & Groom enjoy together… so Backyard Entertaining, BBQing & Grilling, Camping, Home Renovation, Wine, Travelling etc
Guests tend to be couples… again around the couple`s ages… so often the party is thrown by a friend / peer
As for Etiquette (resident etiquette snob here)
The Shower your Mom has planned (as you describe it here) has some major Etiquette issues.
First and foremost, as the MOB, she should not be Hosting the party… as it looks like she is trying too hard to score you stuff (sister, aunt, cousin… they area all ok as Hosts)
Second, Guests at the Shower should be also folks coming to the Wedding. Invites should not be extended beyond that circle (otherwise, again it looks gift grabby)
There are exceptions in that if the Bride belongs to a social group, they may decide to throw a Shower in her honour. These social groups could be… Work – a Club the Bride Belongs to – a group of Neighbours who knew her since she grew up – OR the Church Ladies at her congregation. In all of the cases tho, the Host would be one of those members. And the Invites would only be inclusive to the members of the group
So your Mom wanting to cross boundaries here, and invite others who aren’t Invited to the Wedding, but part of one of these social groups would be incorrect (poor etiquette)… the reason being that it puts them on the spot when the conversation turns to the Wedding (which Guests will do) as they aren’t included in that. So they appear as out-casts. BEYOND RUDE
Your Mom if she really wants to help, should get herself a copy of a good Etiquette Book… there will be many occasions in the planning process where it will come in handy. And as the MOB, it will help her know exactly what her role is as far as Hostessing, and how to do it with elegance & flair. Nothing classier than a MOB who understands how this stuff works, and can make her daughter proud, IMO.
Hope this helps,