You absolutely mustn't support your mum unless you are really very wealthy!
If you have 2 kids at uni in the future, then you will be expected to pay their living costs yourself, which will be around £10k per year, each, for 3 years. SO that's £60k. What if one wants to do a masters? Or changes course and has another year? Another £10k.
If you have anything spare after that, then you should be putting money in junior S&S ISAs for them to save for a house deposit, as bank of mum and dad is really going to be the only way most young adults get on the housing ladder in the near future (in fact some reports say that's already the case)
What about your pension? You need to make sure you have enough for a good standard of living in your retirement - you need more than you think if you don't have a defined benefit pension from work.
And finally, if there's anyhting left over, contribute to a junior pension for each of your kids - then when they work they can start paying into immediately without having to decide whether to, where, how much, what funds.....
On the other hand, your mum has £90k in savings which will last a while.
She'll have her state pension - which could be above the minimum. If it's below the minimum she'll get pension credits and housing support, warm home fund etc. If her health deteriorates, she can get attendance allowance.
She might have to move, but that's normal for elderly people. As mobility deteriorates most people want something smaller and low maintenance anyway.
I really don't think you need to worry about her beyond taking her for a nice meal now and then.
If she's expecting more than that, then you should make clear now that with financial responsibility for 2 kids, you can't afford another dependant.