Raj is funny and his sister is not attractive in any way
She may be a little primly presented, but she is hot. Whether or not you find her attractive is neither here nor there, and your problem, but to say she isn't attractive in any way is
wrong.
The main problem is the 3 girlfriends don't have the comedy chemistry that Penny brings. Her and Sheldon are by far the funniest together but she has her funny moments with the rest of them too, less so with Leonard as they are trying to push the awkward ex couples vibe before they eventually get back together. The other 3 girls aren't funny and aren't good together when they do their all girls nights.
Hadn't looked at it quite that way, you are right. The girlfriends not only defeat the main theme of the comedy, but aren't funny at all. They clutter up the comedy, it makes a neat group of four geeks plus penny into four geeks, Penny and a load of unfunny women. Bernadette is quite cute, sexy, but frankly a tertiary kind of character - like a number nine batting two or three places too high. Likewise Amy Farrah Fowler, except she is a godawful character with no redeeming features. Priya or whatever her name is may be hot, but her relationship with Leonard has one night stand comic value only, to annoy Raj but after that it just annoys everyone (this side of the screen)
Disappointing finale, had some funny bits as the comedy can maintain, but the plots are sheering off exponentially towards naff to the power infinity. The episode and series ending scene(s) were too predictable, not really funny, and it is in danger of becoming more what I might call a soap opera with funny overtones. Before it was independent episodes that you could watch individually without the need for continuity, with some characters who make repeated appearances. Now it is becoming an ongoing saga, the Leonard-Penny saga that drags on. Do they not realise on-off-on-off-on-off just put people OFF............?
Some episodes do you even realise they are geeks who can't get a date? One's getting married, another is in a relationship with another girl wanting him and even one who isn't interested at all has a gal pal/partner of sorts. They've pretty much abandoned Sheldon's regime that made them geeks above all others with 'Halo night', classic games night (or whatever that was called) etc There simply isn't enough time in a 23 or whatever minute episode to cover all the funny things and involve a main set of characters hitting on double figures not including relatives, colleagues etc who make guest appearances now and then.
Methinks it may end in a big bang as the plot explodes out of control.