@LyndaButterfly: There are some big etiquette problems in all of this, and there are some relationship problems and some self-care problems, too. I am very sorry that your brother and his fiancee are hurting you. I am apalled that they are choosing to snub your mother; and worried about your brother in a relationship that seems to involve his surrendering to his partner control over his owh relationships.
The snubs to you, though unpleasant, have the grain of an excuse. The bride has no obligation to have you as a bridesmaid as you yourself say, and a groom must be willing to stand strong against convention in order to ask his sister to be a groomswoman. A verbal invitation is still an invitation and is perfectly polite for a casual laid-back affair. And, although wedding websites have recently started to proclaim that “anyone in a relationship” must be invited with their partner, strict etiquette’s must-invite rule actually applies only to the relationship of marriage. Spouses must be invited together because they are married. Fiances must be invited together because they have agreed to become married. Living-together couples must be invited together because they are living as if they were married (and in some jurisdictions that confers the common-law status of marriage.) While you and your boyfriend have indeed agreed to become married (and that is the definition of “engaged”) and may live together (you haven’t said), your insistance that you are not yet “officially” engaged despite meeting the actual definition of the term, has given the bride a valid reason for not inviting your boyfriend.
Your mother’s offers, of a reception and a shower, were kind and generous. I do understand the bride’s turning down the offer. How many times have we read, on this wedding board or another, or in a magazine column, that “if you want to be able to decide your wedding plans for yourself, then don’t accept anyone’s help in paying for it,” and “close relatives of the couple shouldn’t host showers”. If that is what is motivating the bride to decline your mother’s offers I do understand — but I don’t really approve. Greed for control over every decision is just as ugly as gift-grubbing greed or any other kind of greed. There is also an etiquette rule that, since the reception is the last party planned for a bride before she becomes joined into her husband’s family, that letting the groom’s family give the reception would suggest that she is ashamed of her own family roots and cannot wait to sever them. That is why the bride’s mother and her husband have the first right to give the receptioon if they choose.
But, since generous grooms’ mothers’ like your mother feel shut out by that rule, etiquette grants them the right to host the rehearsal dinner. Will your brother and future sister-in-law be having a rehearsal? Did your mom think of offering to host that, too? If the bride turned that down, then I find myself increasingly annoyed by the bride’s ungraciousness!
As for the shower, the proper rule states that close relatives of the couple can host showers specifically when the shower guests are equally-close members of the family. When the groom’s mother hosts a groom’s-family shower, it functions as a looking-over ceremony, where the family matriarchs get to form an impression about this bride who is marrying into their circle. It is often absolutely terrifying for the bride; but grown-ups face their fears and buck-up and get through things. Still, since the bride might not know the subtleties about who can and cannot invite whom to a shower, it might be more effective to invite her to “tea, to meet some of Buddy’s aunties and cousins.” And if the bride turns down that, too, she is pretty much sending a message that she want’s nothing to do with Buddy’s family.
If you are planning to go to the ceremony only, please look at your state’s marriage laws and the policy of the venue where they will be getting married. Some states require that the actual ceremony be open to the public. Some churches consider the wedding a church service and have a policy that no believer or seeker can be banned from attending a church service. Some parks will reserve a site for a private event, but still allow public access to the general environs of the event. In those cases, your boyfriend is perfectly entitled to attend the ceremony with or without an invitation. Since you do not plan to attend the reception anyway, your boyfriend can then win points with your mama by taking both of you out to an elegant dinner (and having flowers sent to both of you that you can wear at the ceremony, since there seems to be considerable probability that the bride might “forget” to have a corsage made for the Mother of the Groom.) Having a wonderful time at dinner, with people you love and with flowers to lift your hearts, is part of the self-care solution for you and your mama.
When your own wedding rolls around, remember your mama’s feelings now and listen to her advice and accommodate the role that she wants to play as much as you can. And invite your sister-in-law: not because she is ladylike and hospitable, but because you are.