Could it be a slight
unwitting jealousy having you in competition over your brothers and their attentions?
Or perhaps because you're female she has an affinity with you but not with your brothers, males, whose "mysteries" she can't come to terms with?
Difficulties come between mums and sons at puberty.
Until puberty, mother nurtures sons as she does daughters. She tends their wounds, makes their meals, can cuddle them and deal with their emotions. Then at puberty the male biology changes and things that were ok (between mum and sons) quickly become taboo.
It's difficult if not impossible for a pubescent boy to discuss those changes, his urges, with his mum. He has to "be a man". He can't go crying to his mum quite as easily and his adolescent emotions start to get hidden.
Not so with a girl as there's continuity.....sometimes. Mums understand a girl's changes - i.e. menstruation - even if they have trouble explaining the issues to pubescent daughters. They are closer to a daughter's emotions. In some cases it leads to maternal jealousy - and even suspicion.
So I'd just guess that a group of sons together growing up, having their own masculine things would present mums with a bit of a problem. Mum might think it best to leave them alone....although as Freud pointed out, there is a veiled attraction between sons and mothers. Hence she may see you as a competitor for what attentions are allowable....
all without overt awareness.
Just a thought....
But for sure, you have nothing for which you need be forgiven. Perhaps, over a period of time, ridding yourself of such ideas might change her attitude toward you. You don't say how old you are but if beyond adolescence then you're your own person now.
PS: it's sometimes good to vent.