Hi YWithers,
You and your siblings may each have a Capital Gains Tax liability arising on the disposal of your 50% share of the property.
You and your siblings can use the calculator here:
Tax when you sell property
Your mother will be able to claim private residence relief on her 50% share, so is unlikely to be liable to Capital Gains Tax.
While there are no tax implications from you and your siblings, giving or receiving a cash gift, you and your siblings, may have Inheritance Tax implications, if you die within 7 years of giving the cash gift.
You may wish to speak to Inheritance Tax regarding any Inheritance Tax implications
Rules on giving gifts
And
Inheritance Tax: general enquiries
You may also want to review the guidance here:
How Inheritance Tax works: thresholds, rules and allowances
There may be SDLT payable on the acquisition of a property, in yours and your siblings names, but this can be used to reduce any capital gain arising from a disposal of the property.
Thank you.