In a letter sent to Hellaby shareholders today, Castle said the price offered by Bapcor was "compelling".
The letter, signed by two trustees of the Hugh Green Trust and Maryanne and John, said Castle had accepted the offer because of a "high degree of risk and uncertainty associated with Hellaby's ability to deliver on its growth aspirations".
"Whilst Castle Investments would welcome an increase in the offer price, we would be most disappointed if Bapcor walked away from the offer," the letter said.
"We urge our fellow shareholders, provided you share our view, to join us in accepting this offer as soon as possible."
The letter also said that there were concerns about reports that Hellaby had "characterised the acceptance of the offer as being influenced by family matters".
"We feel that the strong implication to be taken from the various Hellaby public statements is that Castle Investments has only accepted the offer because of matters involving the Green family. This is wrong ... we accepted on the basis that the offer price is above our view of fair value of Hellaby shares," the letter said.
After Hugh Green died in 2012, Maryanne and other members of the family fought for control of his $400 million empire.
The eldest daughter of the businessman and philanthropist had been chief executive of the family-owned property firm, the Green Group, but left in the 12 months before he died.
She won a major legal victory last year when then Chief High Court Judge Helen Winkelmann found Hugh Green had been subject to undue influence from John when Hugh made a series of decisions in the last year of his life.
Justice Winkelmann reappointed Maryanne to Green Group companies and declared she was a trustee of the Hugh Green Trust and Hugh Green Property Trust, which control the business side of the Green Group.
The Court of Appeal last month dismissed a challenge to Maryanne's reappointment.
Castle's letter said: "The trustees and Green family members are united in the belief that the offer represents fair value."
Hellaby's independent valuation from Grant Samuel was $3.60 to $4.12 a share and the company said the offer under-valued the business, especially the automotive unit that Bapcor is interested in.
Bapcor countered by saying the valuation did not include Hellaby's corporate head office costs but it failed in a request to the Takeovers Panel to look into alleged breaches of the Takeovers Code by Hellaby.
Steve Smith, Hellaby's independent chairman, this week said the company "rejects Bapcor's views as without merit".
Hellaby shares closed up 1.54 per cent today at $3.29.
- additional reporting BusinessDesk
