Oh dear, she was embarrassed that he bit another child under her care and made a rash "we won't return" as a way of placating everyone. I can sympathise, but really, how silly. Many toddlers bite, one of the reasons we take them to groups is so that they can be supervised whilst they learn how to play nicely.
He'll soon learn that biting means the end of the game (whether because adults intervene or because other children choose to move away and do something else). Please try not to see this as a huge character problem, lots of toddlers experiment like this. We of course have to try our hardest to prevent other children from being hurt but removing him from the group altogether seems a huge over-reaction.
My DS is 18 months, we haven't had him bite or be bitten (yet! hope we can avoid it) but there's all sorts of things he is having to learn, not to snatch toys etc. He's not a bully in the making, he just doesn't know the rules of playing yet, he sees a toy he wants to play with and goes straight for it. He'll learn, we'll help him to learn, and the other mums at the group I go to are understanding, laugh off my apologies and help him learn by offering him or the other child a different toy that isn't being played with.
It has nothing to do with how your younger child is fed. I can't see why anyone would make that connection. All small babies require lots of attention which a sibling could be put out by. I wonder if your mum just isn't confident about handling the biting situation and found the experience so mortifying that she doesn't want to return. But it doesn't mean you can't ever go back. He's not the first toddler to have ever bit anyone.
How are you feeling? You've got a lot on your plate trying to learn how to mother two young children. I only ask because your worries and guilt about DS becoming a bully and it effecting his life forever, is a type of thinking called "catastrophising" (thinking the worst). I'm sure everyone does it sometimes, but it happens more when we are feeling down. Try to look after yourself, getting as much rest as you can even if you need to rely on others to take over household chores etc.
Do remember to ask your HV or GP for help if you are feeling depressed, they won't judge you and they can help.
Breastfeeding gets easier as your baby gets older. You put in the hours now, and later you get the easiest method of soothing and feeding a baby. You'll be multitasking while feeding before you know it.
Some other ideas for toddler whilst you are feeding - colouring book, sticker books, story books, stick on some music for him to dance to, anything he will be able to occupy himself with for a little while, or something you can do together. And the Cbeebies or a dvd isn't the end of the world.
Its lovely that he is already being a caring big brother, try not to doubt that even if he gets frustrated sometimes.
Take care 
