Home country real property is relevant to a visa application because it's a tie between the applicant and the home country. Ties to the home country are used by visa examiners as a proxy for assessing whether the applicant will leave the UK and not overstay.
The applicant's ownership share of this property will be diminished because she is only one of several owners. In addition, a property owner does not have to be physically present at or near the home country property in order to benefit from its ownership, further reducing the strength of a real property owernship tie to the home country.
In any event, your mother's visa application should be honest and complete. The application should be clear about what amounts are claimed by her if they're less than the funds deposited into her account. And if she says that only a portion of the rent received is hers, she must be ready to explain why she holds other people's money in her account.
Note that the multiple ownership could be disclosed by her in the application or independently discovered by the visa examiner. Visa examiners have lots of sources of information, and if things feel "off" or unusual, the examiner may search for information in addition to what's presented in the application itself. Holding money for others will generate concern and interest and maybe suspicion by the visa examiner.
If the application does not disclose or obscures the multiple ownership, and the multiple ownership is discovered by the visa examiner, her application will surely be denied as she'll be seen to have lied on her application.
If your mom is the rent collector, a much more appealing picture would exist if, every month after collecting the rent, she disbursed each other owners' share to them in a traceable form, i.e., by writing a check to each of the other owners. There would then both be regular deposits into her account, and payments out to the other owners. That's clearer and more reassuring than the current situation.
You and your parents should consider putting this trip off for a time, so that a more transparent and representative accounting system can be put in place, and generate some history. If you do that, there'd be greater clarity in her financial situation, far less to explain, and you will increase the liklihood that her visa application will be successful.