Petunia Dursley was equally unpleasant in her own way, and she couldn't believe this was Lily's sister - and that not only because they looked quite different. Her superficial and arrogant behavior made her quite comparable to some of the more bigoted pure-blood witches. The way she spied on the neighbors and then selectively used the information to play them against each other also spoke of someone unpleasant under the coat of normality.
She could not blame a child of not even two years for his behavior, if anything she was blaming his parents on how young Dudley Dursley was acting. Even if these people miraculously decided to treat Harry like their own son, if this was how they intended to raise him, she would fear for Harry's future. She shuddered what Dudley would be with such a rearing once older, and worse, the horror of Harry becoming like that.
No, she would make it clear to Albus that this was no option, regardless the magical protection possible. Sirius would surely be on her side in the matter.
Right now she was a bit bored. Petunia's fat oaf of a husband had already left for work some hours ago and watching Petunia failing miserably at raising her son properly got old real fast. Finally however, something happened when Petunia left the house with Dudley in a pushchair. In broad daylight the fact that such a young child already was overweight became even more glaring.
At least this means I can get some movement into my limbs. Being a cat meant no one would blink an eye at her presence and many would miss it entirely.
However, she was in for a surprise when Petunia stopped after walking into Magnolia Crescent on the way to the park.
From the distance she could see a removal lorry, with several removal men being busy with carrying furniture into a house. The main attraction however clearly was a confrontation between Petunia and man in his early twenties, with a young woman (his wife?) standing at his side. Feeling this could be important, McGonagall moved closer so she could hear clearly what this heated argument was about that made Petunia so angry.
Finally, she managed to hear what was going on.
"...and I can't believe you! We see each other again after years, I learn you have a son, and the first thing you ask is how I dare to live so close to you? Petunia, get you priorities straight!" the man growled, clearly displeased with whatever Petunia had said.
"Thomas Evans, I hoped to never see you again after you sided with my sister!" Petunia shot back.
Said Thomas Evans snorted. "Don't pull Lily into this. You know how much she hated that your parents favored her so blatantly. I tried to stay out of it until you started to blame her for your parents' errors and began to treat her like she doesn't exist."
"I can't believe you are my cousin; and now I have to endure you living so close by," Petuna huffed, before starting to leave, pushing Dudley's pushchair away.
"Thomas, you never told me your cousin is such a nasty woman," the young woman asked Thomas, the man clearly trying to calm down.
Thomas Evans sighed. "Veronica, first I didn't hear anything from neither Lily nor Petunia for several years, and now I find out that not only did Petunia have a son I didn't know about, but she has turned into a nasty shrew. That's not how I imagined the first day in our new home."
"I thought she would be happy to see family. After all, you told me her parents died mere months ago. I don't get it, your parents were really nice when you introduced me to them and said you plan to marry me," Veronica said, trying to understand Petunia's motivations.
"It's a long story. Come, I'll tell you inside."
Just when it got interesting! Watching them walk inside, McGonagall saw that the kitchen window was ajar. So sneaking to under the window, she listened in. She was very curious to hear what was going on, since Lily had never talked much about her family beyond her parents - even her sister only got sparsely mentioned due to the growing rift between the two.
"So, what's her problem?" Veronica, clearly Mr. Evans' wife, asked.
"To be honest, I don't know all the details, either. Somehow, Lily got accepted into some sort of exclusive boarding school at age eleven, and whatever school it was, it caused their parents to heap all kind of praise on her while neglecting Petunia. Needless to say, Petunia didn't take that well and left the parental hom as soon as she was done with school."
"Sounds like they were kind of a messed-up family and it all fell apart," Veronica observed.
Thomas snorted. "Understatement of the year. Petunia left hating them all and Lily was very unimpressed with her parents' attitude and left as well as soon as she was done with school. I had not heard of either of them until today. It's like Lily has vanished from the face of the planet, as if she never existed."
"Ah. And why was Petunia ready to rip your head off?" his wife asked further.
"She's unable to 'forgive' me my 'sin' of telling her that Lily is hardly responsible for their parents being unfair in playing favourites. I really don't want to talk about her anymore..."
Having heard enough, McGonagall quickly left and made her way to the park a little distance away, where she could de-transform between the trees unseen. Then sitting down onto a nearby bench, she tried to collect her thoughts.