Mine's a bit more complicated.
Im not the oldest, or 3ven second oldest. My mum seemed to focus more on my oldest sister who had mental health problems and complained bitterly about DM doing things like turning up to stay close to DSis and sticking her nose in. Thir was done under the guise of supporting Dsis.
At first I thought she was exaggerating but it became more and more apparently as Dsis got sick that DM had some wildly unrealistic expectations and was sometimes actively winding DS up. She'd also phone other family members, including me, hitching if she felt DSis hadn't been kind enough to her - pointing out that Dsis was an alcoholic with terrible mental health problems wasn't what she wanted to hear.
DSis died. I did my best to go down and support my parents even although travel was difficult at the time, I was working and had young kids myself.
My "reward" for this was for my DM to start complaining about me behind my back, expecting a massive amount including me proving care for my DF and to message me early in the morning and late at night... and I started getting physically and mentally ill myself. When I explained I was ill DM didn't want to hear about it, so I tried to pull back a bit and DM stepped up her behaviour and at one point turned up at my house unannounced, mercifully we were all out.
I've sorted myself out now in a lot of ways. I think the eldest is often a convenient target in terms of being dumped on, but if they're not there then someone else gets targeted.
The Stately Homes thread might be worth a read OP. And dont feel you have to put up with it.