Well......everyone contributed, so I think everyone should get to sign the card. Maybe not everyone contributed equally, but gifts are supposed to be given out of good will, so that's really not conducive to the whole "Well, I contributed 1/4 of the cost, but you only gave 1/7, so you shouldn't get to sign." There's really no way around this, except to ensure front that contributions toward the gift will be on a per-person basis, as opposed to per couple. If they suggest per-couple, then you could *gently* suggest that per-person would be fairer, but if you bring it up twice, then that can be perceived as nagging, ungenerous, etc. I know it's not fair......I'm single too, and sometimes it means paying more than your share towards a gift, or being excluded from "couples-only" gatherings, or inviting a female friend to do something, only to have her insist on bringing her SO, thereby making *you* feel like the third wheel......I actually had a friend call me "selfish" and end the friendship because I hadn't seen her in over a year, and I wanted to spend some time with just her without her fiance.
But then, there are also a lot of perks that come with being single. For example, you don't have to worry about buying Christmas gifts for your SO (or, by extension, his family), you don't have to obsess about finding the perfect Valentine's gift for him either, and also, you get to celebrate Single Pride Day, on February 15th....like All Saints Day is to Halloween. I love Single Pride Day. Instead of Valentines, I make Single Pride Day cards with stars on them for all my single friends, and they think it's the coolest thing ever. Oh, and don't forget about half-price chocolate left over from Valentine's Day. Also, if you're feeling gross and don't want to wear make-up for a day (or, conversely, if you feel like dressing nice for no apparent reason), you don't have to justify it to anyone but yourself. Oh, and you don't have to feign interest in the Super Bowl, or the interminably long hockey playoffs either (I think they're actually longer than the regular season). Sure, I could have a boyfriend if I wanted one, it's not as if guys haven't expressed interest (one in particular has been "expressing interest" since first year), but I don't want one, and I might feel differently ONCE in a while, for about five minutes, and then I just crank up some Ani DiFranco, eat some chocolate, and remind myself how good I have it.
But anyway, I'm getting way off track.....as for the gift, technically, your siblings *invited* you to participate in going in on it for your parents, and technically, it's not quite etiquettely correct to try to change the terms of an invitation....since you're close, you can bring up the equality issue once, but if they don't go for it, then the only thing you can really do is either go along with it anyway, or beg off if you really can't afford it.