A U.S. judge yesterday tentatively appointed the elite Washington law firm of Wilmer, Cutler & Pickering to represent Mary Treadwell, the former director of Youth Pride Inc.

Named as Treadwell's chief defense counsel was former Department of Energy chief of staff Deanne C. Siemer.

Treadwell is charged along with three others of conspiring to defraud the government and low-income tenants of thousands of dollars in federal housing funds through a Pride real estate spinoff, P.I. Properties Inc.

Treadwell, Siemer and attorney John H. Pickering appeared yesterday in a pretrial hearing before U.S. District Court Judge John Garrett Penn, where Pickering told Penn his firm would accept the assignment.

In the hearing, Pickering said a senior associate in his firm, Patricia Douglas, is married to Richard Beizer, head of the U.S. Attorney's Office fraud division, which is prosecuting the case.

Partner Andrew Weissman, Pickering said, was special assistant to former HUD Secretary Patricia Roberts Harris at HUD, and now is counsel to her D.C. mayoral election campaign. Assistant U.S. Attorney William D. Pease said Harris will be called as a government witness in the case against Treadwell.

Also, Pickering said, the firm previously represented P.I. Properties Inc. in a suit against the federal government.

Prosecutors said all three of these connections raise "significant problems" regarding the firm's ability to represent Treadwell without conflicts of interest. They have until Wednesday to review the matter and file any formal objections with Penn.

Pease said prosecutors will interview Beizer, who has overseen the Pride investigation, about what he may have told his wife and whether Douglas should be formally isolated from her firm's participation in the case.

Penn earlier granted Treadwell a court-paid defense after Treadwell complained that she could not afford the fees necessary to pay for the kind of sophisticated legal advice required for the complicated case.

Last month, Penn appointed the prestigious firm of Covington & Burling to represent Treadwell. Covington & Burling withdrew shortly thereafter, however, saying a previous civil law suit its attorneys had handled against Treadwell raised "obvious ethical problems" with the firm's handling her case now.

Also at yesterday's hearing were Treadwell's codefendants, former P.I. Properties officials Charles W. Rinker Jr., Joan M. Booth (Treadwell's sister), and Robert E. Lee.

Penn told Lee that he had accepted a federal magistrate's recommendation that Lee also be granted court-appointed counsel, to be paid for under the federal Criminal Justice Act.

Siemer, Treadwell's new counsel, left DOE in 1980 after a report by the department's inspector general charged that she had violated nepotism rules by placing her stepson in a DOE internship. She acknowledged making "an error in judgment" but denied any wrongdoing.

Energy Secretary Charles Duncan issued a statement saying Siemer had planned to leave DOE even before the allegations were raised, and no administrative action was taken against her.